verdine and other associated authigenic glaucony phosphate facies from the surficial sediments of the southwestern continental margin of india abstract green grains pale green brown infillings of foraminifer tests and brown friable aggregates occur in the coarse fraction of the surface sediments from the southwestern continental margin of india between the periyar river in the north and quilon in the south study of the nature distribution mineralogy and internal structure of these particles resulted in 1 the discovery of a verdine facies and 2 determination of the relationships of phosphate with glaucony and verdine in these sediments two distinct verdine facies associated zones a shelf zone at about 40 m and a slope zone between 100 and 280 m water depth are distinguished verdine grains are abundant up to 30 where biogenic debris dominates in the sediments and are rare glaucony and phosphate predominantly occur as pale green to brown infillings of foraminifers and as phosphatic friable aggregates in the sediments of the terrace at 330 m water depth glauconitic smectite and carbonate fluorapatite are the respective mineral phases the terrace sediments are sandy and often associated with non glauconitized and non phosphatized mollusc shells phosphatized verdine grains occur at 170 m water depth scanning electron microscopy studies indicate that phosphate is in the form of globular to rod shaped bacteria like structures or microbial filaments or both this type of phosphate occurs within the foraminifer tests and as cement in the friable aggregates glauconitized foraminifers are enclosed in phosphatic matrix and phosphate occurs within the glaucony matrix the intergrowth of green clays and phosphatic globules is observed locally in phosphatized glaucony aggregates and also in phosphatized verdine grains the formation of verdine glaucony and phosphate in these sediments is relict the age of their formation is difficult to estimate precisely but appears to be older than mollusc shells dated at 5710 yrs b p planktonic foraminifers from the terrace indicate a quaternary age verdine formation on the shelf and on the slope is diachronous the shelf verdine facies most probably formed during the late pleistocene high sea level stands or during the early holocene the slope verdine may have formed contemporaneously with glaucony and phosphate on the terrace during low sea level stands the terrace most probably acted as a sort of hardground facilitating glauconitization and phosphatization of the sediments upwelling oxygen minimum zone and low terrigenous sediment accumulation provided adequate conditions for phosphatization which was mediated by microbial processes the intergrowth of phosphate with the authigenic clays of verdine and glaucony may be due to the local confinement of the microenvironment which was probably controlled by microorganisms

the missing silica sink revisiting the marine sedimentary si cycle using cosmogenic 32si burial of biogenic silica bsitotal in high sedimentation rate continental margins remains highly uncertain cosmogenic 32si t1 2 140 years can be used to trace the fates of bsitotal postdeposition including as opal bsiopal and diagenetically altered opal bsialtered the latter dominantly authigenic clay bsiclay to determine the magnitude and form of bsitotal storage in coastal sediments conventional operational leaches targeting bsiopal and bsialtered including bsiclay were modified for large scale samples necessary for measurement of 32si the 32si activity was used to estimate total biogenic silica burial bsitotal bsiopal bsialtered in several depositional settings gulf of papua gulf of mexico long island sound and in the previously studied amazon guianas deltaic system in subtropical and temperate regions 32si was detected in both traditional biogenic silica leaches bsiopal and residual authigenic clays traditional bsiopal and modified operational leaches designed to target the most reactive authigenic silicates bsialtered consistently underestimate authigenic clay formation bsiclay and thus the magnitude of bsitotal burial in temperate coastal zones and subtropical deltas by 2 4 fold in tropical deltas 32si activities in the residual fraction after removal of bsiopal demonstrate rapid and almost complete alteration of initial bsiopal to new forms most likely bsiclay globally 4 5 4 9 tmol yr si may be trapped in marine nearshore deposits as rapidly formed clay bsiclay 100 of the missing silica sink in the marine silica budget

provenance shallow to deep diagenesis and chemical mass balance in supermature arenites and pelites ordovician simpson group oklahoma and kansas u s a abstract this study considering original detrital compositions and diagenetic alterations relates sandstone and shale mineral and chemical composition to provenance and mass balance between the sedimentary assemblage and its provenance mature quartz rich sandstones and siltstones with clay rich shales in cores at depths of 60 m 5 2 km and burial temperatures of 150 c from over 50 wells were utilized in this work these provide the material for analysis of compositions and alterations in coastal and shallow marine clastic sedimentary rocks of the middle ordovician simpson group in oklahoma and kansas u s a mineral and geochemical data were obtained by optical petrography x ray diffraction scanning electron microscopy electron microprobe and whole rock x ray fluorescence and icp ms analyses results of the study show significant progressive diagenetic changes with increased burial depth and temperature mineralogically and chemically less altered rocks are comparable to modern quartz rich sands and kaolinitic muds in siliciclastic sediments developed during humid tropical weathering in crystalline basement shield provenance in africa and south america by analogy to modern tropical cases muds composed of major kaolinite with minor smectite and illite at deposition were diagenetically transformed to illite rich rocks by possible microbial action at surface to shallow 20 in sandstone buried 3 km during deep burial high temperature 110 c calcite cement from deep basin sources was precipitated in sandstones geochemical mass balance estimates physical and chemical evidence suggest quartz cement was sourced from within clastic rocks of the simpson group with minor external contribution of sio2 na is in low abundance due to loss in solution during weathering in the provenance rare earth elements ree have typical abundances and distributions in shales with enrichment in light ree littoral environment siltstones and matrix rich sandstones display enrichment of heavy ree in phosphatic brachiopod shell fragments

geological setting of the early devonian rhynie cherts aberdeenshire scotland an early terrestrial hot spring system abstract the drilling of nine cored boreholes including a deep hole 233 m to a gabbroic basement in the vicinity of the rhynie chert locality has resulted in a major revision of the structure and stratigraphy of the area the main new structural element recognized is a low angle extensional fault system which defines the western edge of a half graben containing the early devonian succession the fault system was the main conduit for the fluids that fed the hot springs and a heat source to the southeast of rhynie is indicated rapid and unexpected lateral and vertical variations in lithology together with new lithological units have been identified the latter include a thick unit of intensely altered lapilli tuffs which are unique to the rhynie basin and to other devonian basins in ne scotland analysis of these lithologies in conjunction with the new structural model allows the succession at rhynie to be correlated with the succession in the rest of the basin and a model of basin evolution and hot spring development to be constructed siliceous sinters were deposited by multiple episodes of hot spring activity and are variably interbedded with shales sandstones and minor tuffs in a unit about 35 m thick sinter deposition was ultimately controlled by repeated subsidence along the basin margin geothermal activity was probably widespread in northern britain in the early devonian but the surface deposits are unlikely to be preserved the rhynie deposit has survived due to a fortuitous combination of circumstances

evidence of fluid flow in microfractures in geopressured shales petrographic and microprobe analyses of geopressured shales from the oligocene frio formation brazoria county texas provide direct evidence of hydraulic fracturing and significant fluid flow in the shales the optical continuity of the fracture margins and their higher al si ratios indicate that the fracture margins have undergone more complete alteration of smectite to illite than the interiors of the fracture blocks crosscutting relationships between calcium sulfate and the organic material that fills the fractures indicate that calcium sulfate was emplaced before the organic material and thus was precipitated in the subsurface the calculated fracture permeabilities of the shales on the order of 10 13 m2 are similar in magnitude to that of he frio sandstone reservoirs which suggests that these fractured shales could support flow comparable to that in the sandstones fracture fill deposits or collapse could in turn reduce this flow in addition the more extensive alteration of the fracture margins suggests that fluid in these shales flowed preferentially through the microfractures rather than through the matrix

a comparison of popular neural network facies classification schemes two learning networks are commonly used for seismic facies classification unsupervised and supervised neural networks while unsupervised neural networks have proven to be effective processes for macro and mesoscale depositional facies characterizations published results from supervised neural networks have more often demonstrated effective reservoir scale characterization studies by using neural network mapping rather than neural network classification methods neural networks are sophisticated techniques that reduce data dimensionality and assist in seismic interpretation for exploration exploitation and production projects in essence neural networks for seismic interpretation map multiple seismic attributes to an output attribute or attributes that incorporate combined facets of the input data sets that yield a more accurate subsurface interpretation selecting an appropriate neural network is crucial to maximize interpretation benefits yielding an accurate representation of the subsurface such that it can be used meaningfully to effectively support exploration and production efforts these two classification techniques are compared using oligocene catahoula oil bearing sands overlying a shallow salt structure along the gulf coast where well control is abundant and used to ground truth the seismic classifications of shale brine sands and oil sand extent and thickness extraction of the seismic facies traces at various wells within the 3d volume allows direct comparison of the two techniques with known geology permitting an unbiased evaluation and demonstrating the advantages of supervised neural networks for the classification of pertinent geologies the advantages of the supervised network are demonstrated further by horizon and stratal slices that properly identify productive fault blocks while minimizing false positives of single seismic attributes

carbonate preservation in shallow marine environments unexpected role of tropical siliciclastics abstract coordinated taphonomic geochronologic and geochemical studies of bivalve death assemblages and their sedimentary environments of san blas caribbean panama permit us to identify the major factors controlling skeletal degradation in mixed carbonate siliciclastic tropical shelf sediments ten sites were studied along environmental gradients including water nutrients grain size and sediment chemistry carbonate organic carbon and reactive iron contents taphonomic data were derived from naturally occurring bivalve death assemblages and experimentally deployed specimens of mytilus edulis and mercenaria mercenaria to determine environmental controls on types and intensities of postmortem damage to skeletal hardparts and to quantify short term rates of damage accrual death assemblage shells were dated using 14c and amino acid racemization techniques to examine shell persistence scales of time averaging and long term rates of damage accrual including correlations between shell damage and she

conversion of diatoms to clays during early diagenesis in tropical continental shelf muds in coastal muds downdrift from the amazon river mouth marine diatom frustules are rapidly converted to various forms of authigenic aluminosilicate phases during burial the dominant neoformed crystalline phases have a composition similar to k smectite yield electron diffraction patterns characteristic of clay minerals and exhibit a range of crystal sizes and morphologies including euhedral pseudohexagonal crystals and anhedral flakes replacing biogenic silica a poorly crystalline or microcrystalline k rich and fe rich aluminosilicate material also replaces the siliceous frustules the conversion process is not always complete leaving relics of the original frustule in the resulting authigenic aluminosilicate phases laboratory incubation experiments with cultured diatoms demonstrate that the conversion process occurs in 20 23 months the conversion of biogenic silica to authigenic clays occurs throughout the amazon deltaic deposits and presumably takes place in other comparable depositional settings biogenic silica alteration provides proof for a direct link between the biogeochemical cycle of silica in nearshore environments and the neoformation of cation rich aluminosilicate phases and it may prove to be important for oceanic geochemical cycles as a sink for si k and other elements incorporated in the authigenic aluminosilicates rapid formation of authigenic k smectite may also represent a reaction stage leading to eventual formation of illitic clays during later diagenesis

progress in reconstructing vegetation on the old red sandstone continent two emphanisporites producers from the lochkovian sequence of the welsh borderland small coalified fossils mesofossils have yielded new insights into vegetation of the old red sandstone continent in early devonian times particularly useful are those containing spores that can be placed in dispersed spore taxa although patinate and emphanoid spores have not hitherto been found in situ emphanisporites cf micrornatus richardson lister is described in a bifurcating cylindrical sporangium preserved as a cuticular sheath a terminal dehiscence feature is compared with that in horneophyton the sporangium is encased in amorphous detritus with some tubular fragments similar associations occur on other sporangia e g tortilicaulis and axes at this north brown clee hill locality and they are interpreted as remains of a microbial or fungal film fragmentary cuticles interpreted as isolated sporangial valves bear an undescribed species of emphanisporites with fine interdigitating proximal muri and laevigate distal surfaces referred to emphanisporites sp a richardson lister analysis of dispersed spore assemblages from the locality and others in the welsh borderland indicate that the two emphanoid taxa were not common components of the spore rain this evidence coupled with the dearth of mesofossils of the producers suggests that the plants grew at the upper reaches of the drainage basin of the river that deposited the sediment although the paucity of sporangia may also be attributed to their low fossilization potential

potential salinity driven free convection in a shale rich sedimentary basin example from the gulf of mexico basin in south texas we investigate the plausibility of salinity driven free thermohaline convection in sedimentary rocks of the south texas part of the gulf of mexico basin using salinity data rayleigh number calculations and numerical models previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin but low permeability shales are calculated to be a barrier to free convection in the study area salinity inversions occur either above or within the transition zone from hydropressures to overpressures the positioning of brines over less saline fluids provides a significant buoyancy force rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability 10 15 10 16 m2 numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection simulations indicate that salinity driven free convection can occur at lower permeabilities 10 16 10 18 m2 that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time simple rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems salinity driven free convection at depths in some large sedimentary basins such as the gulf of mexico may be more common than expected

taphonomic trade offs in tropical marine death assemblages differential time averaging shell loss and probable bias in siliciclastic vs carbonate facies radiocarbon calibrated amino acid racemization ages of individually dated bivalve mollusk shells from caribbean reef nonreefal carbonate and siliciclastic sediments in panama indicate that siliciclastic sands and muds contain significantly older shells median 375 yr range up to 5400 yr than nearby carbonate seafloors median 72 yr range up to 2900 yr maximum shell ages differ significantly at p 0 02 using extreme value statistics the implied difference in shell loss rates is contrary to physicochemical expectations but is consistent with observed differences in shell condition greater bioerosion and dissolution in carbonates higher rates of shell loss in carbonate sediments should lead to greater compositional bias in surviving skeletal material resulting in taphonomic trade offs less time averaging but probably higher taxonomic bias in pure carbonate sediments and lower bias but greater time averaging in siliciclastic sediments from humid weathered accretionary arc terrains which are a widespread setting of tropical sedimentation

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

growth faults at the prodelta to delta front transition cretaceous ferron sandstone utah cliff exposures of synsedimentary growth faults at the base of the cretaceous ferron sandstone in central utah represent outcrop analogs to subsurface growth faults delta front sands prograded over and deformed less dense prodelta muds of the underlying tununk shale detailed fault patterns and associated facies changes demonstrate a complex fault history and style for growth fault development rather than a simple progressive development of faults in a basinward position the most proximal and most distal fault sets were the earliest active faults growth faulting was initiated by deposition of cross bedded distributary channel and mouth bar sandstones that reach 9 m thick in the hangingwalls of the faults curvature of the beds in the hangingwall of the faults nucleates smaller conjugate fault sets cross bed sets in the hangingwalls of faults decrease from meter to decimeter scale away from the faults suggesting decreasing flow velocity or decreased preservation of cross sets as a result of decreasing accommodation in distal hangingwalls shifts in depositional loci including upstream and downstream accretion of mouth bar sands contribute to the complex faults history and internal heterogeneity and development of potentially isolated sandy reservoir compartments

an introduction to the rhynie chert the terrestrialization of life has profoundly affected the biosphere geosphere and atmosphere and the geological magazine has published key works charting the development of our understanding of this process integral to this understanding and featuring in one of the geological magazine publications is the devonian rhynie chert konservat lagerstatte located in aberdeenshire scotland here we provide a review of the work on this important early terrestrial deposit to date we begin by highlighting contributions of note in the geological magazine improving understanding of terrestrialization and palaeozoic terrestrial ecosystems we then introduce the rhynie chert the review highlights its geological setting the caledonian context of the rhynie basin and its nature at the time of deposition of the cherts which host its famous fossils there follows an introduction to the development of the half graben in which the cherts and host sediments were deposited the palaeoenvironment this represented and the taphonomy of the fossils themselves we subsequently provide an overview of the mineralization and geochemistry of the deposit and then the fossils found within the rhynie chert these include six plant genera which continue to provide significant insights into the evolution of life on land a range of different fungi with recent work starting to probe plant fungus interactions lichens amoebae and a range of unicellular eukaryotes and prokaryotes algae and cyanobacteria and finally a range of both aquatic and terrestrial arthropods through continued study coupled with methodological advances rhynie fossils will continue to provide unique insights into early life on land

depositional tectonic and eustatic controls on hydrocarbon distribution in divergent margin basins cenozoic gulf of mexico case history abstract the evolution of continental margins and related distribution of hydrocarbon reservoirs in divergent margin basins reflects the primary interaction between sediment supply and process regime within the basin and secondary modifications by sea level change and structural history cenozoic sequences of the northwestern gulf coast basin contain numerous highly explored hydrocarbon fairways that reflect typical productive styles resulting from progressively reduced influences of tectonic events in the north american plate and progressively increased influences of eustasy most paleogene and lower neogene sequences record moderate to high rates of sediment supply locally enhanced load or salt induced subsidence and infrequent relatively low amplitude eustatic change accommodation space was created by crustal loading and gravity driven continental margin extension or salt evacuation along with progradation into the deep water gulf shorezone deposits are sand rich because slow or limited eustatic shoreline shift promoted effective reworking and shelfal storage of coastal facies progradational depocenters were characterized by large long lived wave reworked shelf edge delta systems sands were largely stored close to the delta front and flanking shorezone systems prolonged periods of aggradation and retrogradation preserved thick vertically stacked parasequences of deltaic and interdeltaic barrier strandplain facies hydrocarbons accumulated in multiple superimposed reservoirs primarily in mixed structurally and depositionally defined plays in contrast upper neogene and quaternary sequences are products of frequent high amplitude eustatic cycles rapid subsidence and locally very high rates of sedimentation accommodation space was created by load induced subsidence modified by salt evacuation and intermittent relative rises in sea level shorezone deposits are heterolithic because rapid subsidence and frequent eustatic shoreline shifts prevented extensive reworking progradational episodes were characterized by mud rich river dominated shelf edge deltas of moderate size rapid lowering of sea level promoted gravity remobilization of deltaic sediments and deposition of sand rich lobes that created broad structurally and bathymetrically complex submarine fans and slope aprons some sand rich submarine fan lobes extend more than 160 km basinward from the paleoshelf margin in contrast strike reworking and sand storage on the shelf were minor combination structural and stratigraphic traps define most upper neogene and quaternary hydrocarbon plays correlating observed patterns of cenozoic basin fill with contemporaneous plate interactions and eustatic fluctuations reveals the relative control of these variables on rates of sediment supply characteristics of depositional sequences and styles of basin fill deformation

sulfur and iron diagenesis in temperate unsteady sediments of the east china sea inner shelf and a comparison with tropical mobile mud belts mmbs redox cycling of iron fe and sulfur s exerts profound influences on fates of numerous elements in coastal marine sediments in this study s and fe cycling and its geochemical expressions in the east china sea ecs inner shelf a representative of temperate mobile mud belts mmbs were characterized and compared with tropical counterparts the amazon shelf and the gulf of papua fe and s speciation consistently points to the dominance of authigenic nonsulfidized fe ii phases i e poorly crystalline clays pccs and carbonates and the prevalence of fe redox cycling in the suboxic or weakly sulfidic regimes of the ecs mmbs high contents of authigenic magnetite may be a common diagenetic expression in all mmbs compared to the tropical mmbs three main differences of diagenetic expressions in the ecs mmbs are i light 34spyrite in the ecs mmb vs characteristically heavy 34spyrite in the amazon shelf mmbs ii lower total reactive fe fetr total diagenetic fe ii and ratio of fetr to total fe in the ecs mmbs iii fe ii carbonates and pccs are equally important sinks for nonsulfidized fe ii in the ecs mmbs whereas pccs are the predominant sink in the tropical counterparts these differences are ascribable to factors including low degradability of organic matter small diffusion scales less intense chemical weathering in the drainage basin and or weaker reverse weathering in the ecs mmbs despite the differences above fe and s diagenetic expressions that characterize the prevalence of fe redox cycling in the unsteady suboxic regimes are shared by the ecs and tropical mmbs

40ar 39ar dating of hydrothermal activity biota and gold mineralization in the rhynie hot spring system aberdeenshire scotland this study presents a new high precision 40ar 39ar age for the devonian hot spring system at rhynie hydrothermal k feldspar sampled from two veins that represent feeder conduits and a hydrothermally altered andesite wall rock date the hydrothermal activity the fossilised biota and syn k feldspar gold mineralization at 403 9 2 1 ma 2 oxygen isotope data for the parent fluid 4 to 2 show that the k feldspar was precipitated from a dominantly meteoric fluid which mixed with magmatic fluids from a degassing magma chamber the 40ar 39ar age 403 9 2 1 ma 2 when recalculated 407 1 2 2 ma 2 with respect to the astronomically tuned age for fish canyon sanidine 28 201 0 023 ma 1 also provides a robust marker for the polygonalis emsiensis spore assemblage biozone within the pragian earliest emsian furthermore the age identifies the devonian pull apart volcano sedimentary basins of the british and irish caledonides and their root zones as specific targets for future gold exploration

cenozoic evolution of sediment accumulation in deltaic and shore zone depositional systems northern gulf of mexico basin abstract paleogeographic and volumetric lithofacies mapping of 18 cenozoic genetic sequences within the northern gulf of mexico basin quantifies the proportional sequestering of sediment within wave dominated shore zone vs deltaic systems through time three long term depositional phases are revealed by plots based on paleogeographic and sediment isochore maps of total shore zone system area and volume to total delta system area and volume sz d 1 sz d area and volume ratios are highly variable in paleocene through eocene sequences however typical volume ratios for major genetic sequences upper middle and lower wilcox queen city qc and yegua range between 0 2 and 0 6 minor sequences sparta sp jackson js which record very low rates and volumes of sediment accumulation have the greatest variability in their ratios 2 oligocene and miocene sequences display consistently high proportions of shore zone sediment sz d area ratios range from 0 6 to 1 0 and volume ratios cluster between 0 4 and 0 8 3 a substantial late neogene decrease in sz d ratios is presaged in the late miocene sequence pliocene and pleistocene sequences are uniformly characterized by very low ratios of

diagenesis in shales from a partly overpressured sequence in the gulf coast texas usa abstract argillaceous rocks from a partly overpressured tertiary sequence in a texas gulf coast offshore well have been examined using x ray diffraction xrd back scattered electron microscopy bsem and electron microprobe analyses the xrd data show that there is increasing illitization of illite smectites at depths below 2300 m the bsem and microprobe data indicated that the illitization occurred in two microenvironments the matrix and cavity infills in the matrix the illitization appears to be a transformation process and in the cavity infills by neoformation other diagenetic processes identified by the bsem analyses include the formation of kaolinite and illite in cavity infills and chlorite in the matrix and cavity infills calcite cementation has also been observed along with k feldspar dissolution and mica transformation these observations indicate the complex and heterogeneous nature of the diagenetic processes in these argillaceous sediments the presence of overpressuring reflects a combination of processes initial undercompaction arising from rapid sediment deposition illitization of illite smectites calcite cementation creating permeability barriers the generation of hydrocarbons and faulting may also have been contributory factors to the overpressuring pattern now observed

chronostratigraphy and tectonostratigraphy of the columbus basin eastern offshore trinidad the columbus basin forming the easternmost part of the eastern venezuela basin is situated along the obliquely converging margins of the caribbean and south american plates the two primary structural elements that characterize the basin are 1 transpressional northeast southwest trending anticlines and 2 northwest southeast oriented down to the northeast extension normal faults the basin was filled throughout the pliocene and pleistocene by more than 40 000 ft 12 200 m of clastic sediment supplied primarily by the paleo orinoco delta system the delta prograded eastward over a storm influenced and current influenced shelf during the pliocene pleistocene depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower pliocene to pre pliocene mobile shale facies biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two dimensional and three dimensional seismic data to construct a chronostratigraphic framework for the basin as a result several observations were made regarding the basin s geology that have a bearing on exploration risk and success 1 megasequences wedge bidirectionally 2 consideration of hydrocarbon system risk across any area requires looking at these sequences as complete paleofeatures 3 reservoir location is influenced by structural elements in the basin 4 the lower limit of a good quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location 5 reservoir quality of deep marine strata is strongly influenced by both the type of shelf system developed bypass or aggradational and the location of both subaerial and submarine highs and 6 submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure migration and trapping of hydrocarbons and the distribution of hydrocarbon type begin page 1906

mineral water interaction fluid flow and frio sandstone diagenesis evidence from the rocks the spatial distribution of burial diagenetic cements in sandstones of the frio formation is related to permeability and fluid flow the relationship between sandstone diagenesis and fluid flow implies that the modification of the rocks is largely the outcome of reactions between the detrital and authigenic minerals and the formation water in the pores during burial formation water preferentially flowed through the most porous and permeable sandstones diagenetic alteration is more extensive in permeable sandstones because the minerals in those rocks were exposed to significantly more pore volumes of reactive water each of which accomplished a small amount of diagenetic work consequently porous and permeable units were also the preferred sites of diagenetic mineral p ecipitation the heterogeneous distribution of authigenic minerals in sandstones is a result of changing preferential flow paths due to diagenetic modification during burial sandstones that are not the most porous or permeable at the time of deposition commonly preserve the best reservoir quality at depth because they are less modified by diagenesis petrographic analyses indicate that the sio2 needed for the quartz overgrowths in the sandstones had an allochthonous source dgr18o values for diagenetic quartz imply that the mineral precipitated from rapidly ascending hot formation water quartz overgrowths are most abundant in distal shelf facies sandstones where the isolated nature of the sandstones in a predominately shaly section maximized preferential fluid flow mass balance calculations show that thousands of pore volumes of formation water are required to supply the sio2 needed for the quartz overgrowths in the sandstones the amount of water required to produce the diagenetic modification of the sandstones is much larger than the amount of water present in the entire frio formation impl ing that reuse of formation water is an important and necessary process during diagenesis

co diagenesis of iron and phosphorus in hydrothermal sediments from the southern east pacific rise implications for the evaluation of paleoseawater phosphate concentrations abstract we present a detailed study of the co diagenesis of fe and p in hydrothermal plume fallout sediments from 19 s on the southern east pacific rise three distal sediment cores from 340 1130 km from the ridge crest collected during dsdp leg 92 were analysed for solid phase fe and p associations using sequential chemical extraction techniques the sediments at all sites are enriched in hydrothermal fe oxyhydr oxides but during diagenesis a large proportion of the primary ferrihydrite precipitates are transformed to the more stable mineral form of goethite and to a lesser extent to clay minerals resulting in the release to solution of scavenged p however a significant proportion of this p is retained within the sediment by incorporation into secondary goethite by precipitation as authigenic apatite and by readsorption to fe oxyhydr oxides molar p fe ratios for these sediments are significantly lower than those measured in plume particles from more northern localities along the southern east pacific rise and show a distinct downcore decrease to a depth of 12 m molar p fe ratios are then relatively constant to a depth of 35 m the fe and p speciation data indicate that diagenetic modification of the sediments is largely complete by a depth of 2 5 m and thus depth trends in molar p fe ratios can not solely be explained by losses of p from the sediment by diffusion to the overlying water column during early diagenesis instead these sediments are likely recording changes in dissolved p concentrations off the sepr possibly as a result of redistribution of nutrients in response to changes in oceanic circulation over the last 10 million years furthermore the relatively low molar p fe ratios observed throughout these sediments are not necessarily solely due to losses of scavenged p by diffusion to the overlying water column during diagenesis but may also reflect post depositional oxidation of pyrite originating from the volatile rich vents of the southern east pacific rise this study suggests that the molar p fe ratio of oxic fe rich sediments may serve as a proxy of relative changes in paleoseawater phosphate concentrations particularly if fe sulfide minerals are not an important component during transport and deposition

primary siliceous rhizoliths from loop road hot springs north island new zealand abstract siliceous rhizoliths and rhizocretions which developed in diatomaceous sediments at loop road hot springs on the north island of new zealand started to form while the plants were alive or shortly after their demise the porous and solid laminae in the rhizoliths and rhizocretions incorporate well preserved filamentous microbes spores and diatoms the silica needed for the formation of these laminae came from the hydrothermal fluids that permeate through the sediments silica precipitation was triggered by degradation of the plant tissue which caused a local increase in co2 and a decrease in ph during the initial phase of rhizolith development the plant tissues acted as a template for silica precipitation the silica precipitated during these early phases provided the sites for later more extensive silica precipitation

sources of iron for pyrite formation in marine sediments more than two hundred aerobic continental margin aerobic deep sea dysaerobic and anaerobic euxinic sediments have been examined for their variations in different operationally defined iron fractions each of which represents a different reactivity towards dissolved sulfide aerobic continental margin deep sea and dysaerobic sediments contain similar contents of highly reactive iron dithionite soluble iron plus pyrite iron poorly reactive iron iron soluble in hcl less that soluble in dithionite and unreactive iron total iron less that soluble in hcl by contrast non turbidite euxinic samples from the black sea as well as euxinic samples from the cariaco basin and framvaren are enriched in highly reactive iron these sediments contain a small lithogenous fraction and a large biogenous organic c rich fraction which decays by sulphate reduction in an iron rich water column to form pyrite rich sediment other anaerobic euxinic samples from the black sea orca basin and kau bay contain lower concentrations of biogenous sediment and are not therefore enriched in highly reactive iron degrees of pyritization dop for all the aerobic dysaerobic and anaerobic euxinix samples except those low in biogenous material are consistent with analogous ancient sediments and indicate that most pyrite formation occurs form the highly reactive iron fraction

a geothermally influenced wetland containing unconsolidated geochemical sediments hot spring waters flowing from porkchop geyser norris geyser basin yellowstone national park usa enter a shallow wetland basin and precipitate opal a silica particulate particulate formation by chemical rather than biochemical and colloidal mechanisms is suggested by floc and shard like particle morphologies comprising opal a silica nanospheres and microspheres of colloidal dimensions and precipitation from waters with opaque milky blue colouration indicative of aqueous silica sol conditions sediment accumulates in the wetland at a rate of ca 20 25 mm year is unconsolidated and massive to diffusely bedded to laminated post depositional features include soft sediment deformation and scouring and in drying conditions relatively deep desiccation establishment of geochemically dominated wetland sedimentation is favoured where alkali chloride hot spring fluids of circum neutral to basic ph and high silica concentration discharge to and cool to ca 35 c within topographic depressions that receive only small volumes of non hot spring water local wetland vegetation which is composed of hydrophytes halophytes and alkali tolerant species more typical of coastal wetlands colonizes the soft wetland substrate and may be relatively quickly buried by rapid sediment accumulation prior to the evolution of the diatom silica sink geothermal wetlands containing geochemically precipitated silica sediments may have been much more common and widespread rhizoliths chert nodules with organic cores scour fabrics soft sediment deformation desiccation cracks and massive to diffuse bedding preserved in palaeozoic geothermal environments may all be evidence of ancient unconsolidated geochemical sediments and geothermal wetland conditions

the vicksburg formation of texas depositional systems distribution sequence stratigraphy and petroleum geology the lower oligocene vicksburg formation of the gulf coastal plain contains major petroleum reservoirs in the rio grande embayment and is an economically viable target in other areas of texas knowledge of the distribution of vicksburg depositional systems is essential to understanding sandstone concentrations and therefore is fundamental to effective exploration and production of the vicksburg section the depositional setting of the vicksburg reservoirs their position in a sequence stratigraphic framework and the influence these factors have on the petroleum geology of the vicksburg are the focus of this paper surface and subsurface geological and geophysical data provided the framework for an analysis of the depositional systems and the petroleum geology of the vic sburg the two primary texas vicksburg depocenters the rio grande embayment and the houston embayment are separated by the san marcos arch a deep rooted structural nose within the embayments sand rich deltaic complexes merged along strike with barrier strand plains contemporaneous growth faulting controlled depositional patterns of shelf edge deltas in the rio grande embayment but had only a minor effect on the configuration of the shelfal deltas in the houston embayment smaller wave dominated shelf delta complexes interspersed with barrier strand plains extended across the san marcos arch updip of these sandy paralic depocenters fluvial systems traversed mud rich coastal plain units seaward of the paralic systems sand and mud deposits prograded across and built up over the r lict jackson shelf and shelf margin shales these depositional complexes are contained in the systems tracts of one eustatic exxon sequence vicksburg production from each of the three structural regions of texas is characterized by reservoirs from different systems tracts and of distinct different depositional origins

petroleum generation in the southeast texas basin implications for hydrocarbon occurrence at the south liberty salt dome geochemical characteristics of hydrocarbons from the south liberty field liberty county texas were integrated with local stratigraphy pressure temperature seismic data and formation water chemistry to determine the source maturity and migration pathways of hydrocarbons associated with the salt dome in the southeast texas gulf coast fourteen crude oil samples from the soft geopressured cook mountain and yegua both eocene and frio oligocene reservoirs 989 2886 m 3245 9469 ft were analyzed by whole oil high resolution gas chromatography gc and gc mass spectrometry pressure and temperature data from 36 wells were used to model the thermal maturation history in the vicinity of the dome south liberty oils were found to belong to a single genetic family sourced from rocks with a similar level of maturity biomarkers indicate they were formed in a marginal marine environment with notable terrestrial input from a likely lower tertiary source rock probably the downdip lower claiborne and or wilcox group oils were generated within the peak oil window vitrinite reflectance 0 6 0 9 at expulsion temperatures of 125 to 130c thermal modeling indicates the lower claiborne and or upper wilcox beds attained optimum maturity in and around the dome between 8 to 20 ma and 18 to 31 ma respectively oil migration toward the structure probably occurred through faults breaching the deep seated lower claiborne upper wilcox source two severely biodegraded samples were found and these were associated with cooler formation water 50 to 55c with a greater meteoric water component 48 60 comparison with brazoria county oils 100 km 62 mi to the southwest indicates that the south liberty oils belong to the same genetic family with minor differences resulting from a greater input from terrestrial kerogen

subaqueous silicification of the contents of small ponds in an early devonian hot spring complex rhynie scotland the early devonian rhynie and windyfield cherts of northeast scotland originated as siliceous sinters deposited by hot springs silicification affected both subaerial and subaqueous environments preserving a diverse terrestrial and freshwater biota cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content textures comprise clotted and microcoprolitic textures bacterial coatings on plant axes that can be stromatolitic and a variety of bacterial and fungal meshworks the crustacean lepidocaris the euthycarcinoid heterocrania the charophyte alga palaeonitella and chytrid fungi are typical elements of the aquatic biota observations of modern hot springs in yellowstone national park wyoming u s a demonstrate that shallow ponds and streams on low angle outwash areas and dormant vent orifices provide suitable environmental analogues textures comparable to those described from rhynie are recorded from yellowstone sinters but examples of the

early diagenetic alteration of lateritic particle coatings in amazon continental shelf sediment abstract intense chemical weathering in the tropical amazon river basin strips crustal material of alkali and alkaline earth metals leaving a residue composed predominantly of oxyhydroxides of fe hydroxides of al and cation poor clays this lateritic residue often coats soil particles within the marine environment lateritic particle coatings on sand undergo or have the capacity to undergo rapid reaction and mineralogical alteration 1 at least 20 of fe in coatings can be readily liberated during short term incubation in anoxic sediment 2 coatings change color from red white to green yellow during burial implying fe3 fe2 3 average mg cation content of coatings increases net addition from 1 85 near the amazon s mouth to 4 76 120 km from the mouth or with burial 4 a narrowing of the measured range and a slight increase of si al occurs near mouth to offshore 5 mg fe al layer silicates are probably forming by transformation of kaolinite or other cation stripped material of similar composition in coatings mg uptake by lateritic particle coating reactions assuming comparable involvement of all particle sizes could account for up to 67 of the mg discharged in solution by the amazon river evidence for mg fe al layer silicate formation supports the hypothesis that ancient ironstone minerals formed in some cases by direct alteration of lateritic material

taphonomic signatures in molluscan fossil assemblages from the holocene lagoon system in the northern part of the coastal plain rio grande do sul state brazil abstract molluscan shell accumulations shell beds are very common in shallow marine and estuarine environments in south america and also on the continental shelf between argentina and southern brazil the development of these shell beds is restricted to the quaternary and can provide a great opportunity for taphonomic studies which are rare in this geographic location this study investigates the influence of environmental processes on the destruction of biological remains in a subtropical lagoonal setting by describing the taphonomic signatures occurring in the mollusk shells from the tramandai lagoon northeastern rio grande do sul state brazil this lagoon system originated during the regressive phase that followed a transgressive maximum at 6 0 6 5 ka bp the taphonomic variables fragmentation margin modification surface alteration hinge fine scale alteration and corrosion degree and their damage states were recorded for 1000 shells of erodona mactroides the shells were recovered from two outcrops 8 km apart 14c ams dating were performed on the shells of e mactroides from those outcrops which yielded similar ages from 1060 to 1490 cal yrs bp the ages would indicate that the tramandai lagoon occupied a larger area and must have been subjected to very rapid progradation towards its present position several taphonomic variables such as fragmentation surface alteration and corrasion extent and oxidation presented significant differences p

a devonian auriferous hot spring system rhynie scotland the early devonian rhynie hot spring system is the oldest known and is of the low sulphidation type it extends for at least 1 5 km along a major fault zone defining the western margin of an outlier of fluvial and lacustrine sediments plant bearing sinters and andesitic lavas the age of sedimentation and hydrothermal activity has been determined by palynological pragian and radio metric 396 12 ma techniques the outlier is a half graben with a complex stepped western margin the devonian rocks show intense hydrothermal alteration along the fault zone the main alteration minerals are quartz k feldspar calcite hematite and illitic and chloritic clays multiple chert veining and brecciation are widely developed and geyserite and vent material are also present pyrite occurs in veins and all alteration facies sinters and altered rocks contain high concentrations of au as sb hg w and mo gold occurs in arsenian pyrite and as sub micron particles in oxidized rocks the fluid s responsible for most hydrothermal alteration were near neutral with low sulphur and oxygen activities and dominated by meteoric water however incursions of high temperature 300 440 c magmatic fluids occurred with d 65 and 18 o around 8 5 34 s pyrite and initial 87 sr 86 sr ratios vein calcite lie mainly within the ranges 3 4 to 8 5 and 0 71138 to 0 71402 respectively these data indicate that late proterozoic dalradian metasediments are a likely source for s and sr but other sources are possible 13 c values for caliche and vein calcite imply derivation of carbon from non organic sources the rhynie cherts were deposited from a low salinity fluid of probable meteoric origin 18 o chert 13 1 to 16 5 which had interacted with the basement rocks and sediments high xe ar br cl and i cl ratios plant bearing chert yielded an 40 ar 39 ar ratio 292 1 0 6 significantly less than that of modem air and may be the first valid determination of a sample of ancient atmosphere

the enasen gold deposit central sweden stable isotope analyses of quartz sulphides and magnetite were conducted to provide information on thermal history and source of hydrothermal fluids in the palaeoproterozoic enasen gold deposit reequilibration and homogenization of oxygen isotopes throughout the rock have apparently not occurred despite the upper amphibolite to granulite facies regional metamorphism that has affected the rocks however oxygen isotope geothermometry on a coexisting quartz magnetite pair gave a minimum temperature for peak metamorphism of around 650 c which agrees with fe mg geothermometry this suggests that grain scale equilibrium is achieved the variation in oxygen isotope ratios 18o 7 3 10 5 on quartz from the metamorphosed acid sulphate alteration zone is suggested to represent a cooling trend in the fossil hydrothermal system with higher 18o values in more superficial parts temperatures of alteration and silicification and isotopic composition of hydrothermal fluids could not be defined from the present data but it was recognized that the data is compatible with a epithermal genesis for the deposit it is suggested that alteration silicification and mineralization at the enasen gold deposit took place in a high sulphidation epithermal environment at temperatures of around 200 250 c and that the hydrothermal fluids consisted of meteoric and magmatic water a tentative reconstruction of the fossil hydrothermal system is presented sulphur isotope ratios of sulphides from the fold bearing quartz sillimanite gneiss gave 34s values close to zero indicating a magmatic source of the sulphur

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

verdine and glaucony facies from surficial sediments of the eastern continental margin of india the present paper reports the first mineralogical description of green grains verdine and glaucony from sediments of the eastern continental margin of india only 24 of the 82 sediment samples studied at depths between 18 and 247 m contain green grains and their percentages in the coarse fraction 125 250 m range from 6 to 40 of these 13 samples from the inner shelf 18 70 m contain irregular dark green grains 6 samples from the outer shelf 70 125 m contain dark green pellets and a few dark green internal moulds of skeletal materials and 5 samples from the continental slope 160 247 m contain dark green moulds and infillings of planktonic and benthic foraminifers the green grains found at depths between 18 and 125 m consist of poorly crystalline phyllite v with very little detrital clay at 170 m depth odinite young phyllite v occurs and further seaward between 200 and 247 m glauconitic smectite is present all verdine and glaucony grains studied show an early stage of evolution and the colour does not reflect their evolution the verdine and glaucony from the slope sediments may correspond to the last glacial maximum and the outer shelf verdine facies formed during the subsequent transgression the inner shelf green grains may represent the present day formation of verdine

nature of source material for hydrocarbons in cooper basin australia the cooper basin which produces a large proportion of australia s natural gas also includes small oil fields source rocks for the hydrocarbons are believed to be the terrestrial coal measure sediments of the permian gidgealpa group dispersed organic matter in the sediments of the gidgealpa group in the patchawarra trough cooper basin ranges between 1 and 5 by volume through seven formations less than 10 of the dispersed organic matter is exinite and less than 30 is vitrinite if these two macerals only are considered as source material for the liquid hydrocarbons none of the formations contains the 0 5 organic carbon currently deemed necessary for a sediment to be classified as a source rock if inertinite is regarded to have generating potential then most of the formations rate as good to excellent source rocks recent studies on vitrinites and inertinites from a variety of coal seams suggest that if vitrinite is accepted as a possible source of liquid hydrocarbons then inertinite is equally prospective consequently dispersed inertinite in the gidgealpa group sediments is proposed as one of the sources for the liquid hydrocarbons in the cooper basin

the miocene atastra creek sinter bodie hills volcanic field california and nevada 4d evolution of a geomorphically intact siliceous hot spring deposit abstract the atastra creek siliceous hot spring deposit or sinter occurs in the paramount bald peak alteration zone due north of the bodie precious metals mining district in the miocene bodie hills volcanic field california and nevada u s a distinctive features include its geomorphically intact geyser vent mounds the presence of growth fault stepped sinter terraces that developed westward along a nnw trending fault and atypical hg sb and as concentrations high temperature 75 c subaerial proximal apron sinter occurs in two settings 1 radiating botryoidal geyserite in vent mounds and 2 bedded geyserite columnar nodular size sorted geyser eggs intercalated with mid to low temperature

diagenetic transformations opal a to quartz of low and mid temperature microbial textures in siliceous hot spring deposits taupo volcanic zone new zealand silica sinter is a subaerial hot spring deposit formed upon cooling 100 c of discharging alkali chloride waters silica deposition traps and fossilizes living microbes in low temperature 35 c to mid temperature 35 59 c apron terrace outflow channels and pools which record distinctive macrotextures and microtextures along a thermal gradient sinters from four geothermal fields orakei korako northern waiotapu te kopia and umukuri within the taupo volcanic zone new zealand were sampled from two common microbe rich microfacies low temperature palisade mid temperature bubble mat through a range of ages modern to 40 000 years bp we observed morphologic changes in microbial silicification and stepwise transitions in silica phase mineralogy throughout diagenesis opal a to quartz x ray powder diffractometry analysis of taupo volcanic zone sinter samples revealed that mode of microbial fossilization is controlled by silica phase mineralogy which also determines the preservation potent

diagenesis of sandstones at shale contacts and diagenetic heterogeneity frio formation texas diagenetic trends near sandstone shale contacts were studied in 12 cored sequences from four wells between depths of 5200 and 15 700 ft 1585 and 4785 m to evaluate the heterogeneity of diagenetic processes on a local scale and to evaluate the hypothesis that reactive aqueous fluids and components for cements in sandstones were derived from adjacent shales the only evidence suggesting that diagenetic components in sandstones were derived from immediately adjacent shales is an increase in chlorite cement in sandstones toward contacts with shale beds for two of three contacts appropriate for study secondary pores and cements of quartz carbonate and kaolinite do not correlate with proximity to shale beds but have a preference for sandstones that had relatively high ini ial porosities and permeabilities thus the flux of formation water and probably long distance transportation of diagenetic components were more important influences on reservoir quality of sandstones than was the local availability of components multiple regression of 22 independent variables indicates that the best predictors of secondary porosity are kaolinite cement and intergranular porosity sandstone sequences are extremely heterogeneous in the distribution of total thin section porosity secondary porosity and quartz and carbonate cements in addition they have significant variations in the abundance of kaolinite and chlorite cements mass balance calculations for silica and aluminum indicate silica was imported to and aluminum was exported from the sandstones

hydrochemical constraints on fluid mineral equilibria during compaction diagenesis of kerogen rich geopressured sediments abstract hydrochemical conditions characteristic of kerogen rich geopressured sediments contribute to the development of a predictable alteration mineralogy the following conditions result in the geopressured fluid becoming enriched and in some cases saturated with co 2 and ch 4 1 1 the upward and generally restricted flow of fluids in geopressured sediments in contrast to the generally more rapid flow of fluids in nongeopressured sediments 2 2 the coincidence of the depths of geopressuring with the geothermal temperatures necessary for co 2 and ch 4 release and 3 3 the opposing rates of sediment subsidence and co 2 and ch 4 transfer into the upward flowing fluid calculation of the phase relations in the systemcao feo mgo na 2 o al 2 o 3 co 2 h 2 s h 2 so 4 sio 2 h 2 o hcl as constrained by the above hydrochemicalcriteria allows prediction of three patterns of mineral deposition during diagenesis of kerogen rich geopressured sediments quartz deposition is favored in the upper portions and margins of the geopressured section and at the contact between geopressures and normal pressures because of increased fluid flux atshallower depths within the geopressured section carbonate deposition could occur above the zone of co 2 release from kerogen degradation as a result of the upward flux of co 2 and ch 4 enriched andpossibly saturated fluids and the subsequent decrease in fluid temperature pressure and co 2 solubility kaolinite carbonate could deposit within and above the zone of co 2 release from kerogen as a result ofsilicate dissolution by co 2 rich acid pore fluids followed by the potential for illite carbonate and albitecarbonate deposition upon co 2 depletion in contrast laumontite and anhydrite should not depositduring diagenesis of kerogen rich geopressured sediments but could deposit during diagenesis of nongeopressured or kerogen poor geopressured sediments these mineralogic relationships compare favorablywith observed relationships in the kerogen rich geopressured sediments of the frio formation from thetexas gulf coast

seismological studies of a fluid injection in sedimentary rocks east texas induced microseismicity data from a large volume fluid injection into sedimentary rock was analyzed to study the fracture system fluid pathways and state of stress in the lower frio formation in east texas seismicity data are from two arrays of 25 3 component geophone packages sited in two monitoring boreholes from a total of 2 894 event triggers a subset of 54 microearthquakes was chosen for their high quality seismograms and clear p and s arrivals arrival times were picked with a precision of 0 5 to 1 0 ms and microearthquakes were located with hypocentral uncertainties estimated as less than 10 20 m hypocenters farthest from the injection well define a nearly horizontal tube of seismicity approximately aligned in the direction of the injection well a simultaneous inversion of arrival times for transverse isotropic velocity structure and hypocenters yielded p wave anisotropy of 14 and s wave anisotropy of 2 thus velocities along vertical ray paths are higher than those along horizontal paths probably because of lithologic differences single event focal mechanisms were determined for 47 events and many of them are normal fault type the minimum principal stress derived from the focal mechanisms is nearly horizontal and trends approximately north south consistent with the regional stress state an imaging analysis of the seismograms shows the presence of strong seismic scatterers at positions that correlate with boundaries seen in the hypocenters both features probably result from a similar set of heterogeneities this study demonstrates the abundance of information that can be extracted from induced seismicity data and underscores the value of induced seismicity monitoring for studying the fluid and fracture systems created by fluid injections

early to middle devonian ironstone and phosphorite in the northwestern gondwana parnaiba basin brazil a record of an epeiric margin paleoceanographic changes abstract pragian to givetian successions in the caninde group parnaiba basin brazil present phosphorites ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering ocean oxygen levels and black shale deposition events these facies are associated with the evolution of the northern gondwana epeiric sea margin of the rheic ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record we conducted an integrated study based on sedimentology sequence stratigraphy petrography scanning electron microscopy and x ray diffraction to understand the paleoenvironmental stratigraphic and diagenetic controls lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences late eifelian and givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones pragian emsian to givetian ironstones were represented by authigenic coated phosphatic iron silicate odinite chamosite clinochlore francolite and siderite precipitation these are related to normal regressions under lowstand and highstand systems tract oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained fe si and p supply on a storm influenced to storm dominated shelf with an additional source of iron also associated with river discharges phosphorite was formed in a stratified ocean near storm wave base under anoxic sulfide reducing bacteria microbial communities and suboxic conditions productivity fueled by recycled p shallower anoxic conditions near the fair weather wave base existed possible related to a shift in the locus of upwelling and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level ventilation terrigenous or marine contribution and progressive microbial degradation at the beginning of late eifelian transgression an event of increased delivery of continental flux and eutrophication can also be proposed the link between organic rich intervals and third order black shale events daleje chot c and kacak suggests that extinction events can be tracked by analyzing sections in south america global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of c sink conditions probably contributed to the ocean oxygenation

ca mg of brines in miocene oligocene clastic sediments of the texas gulf coast buffering by calcite disordered dolomite equilibria the compositions of brine collected from geopressured and nongeopressured sediments of the northeast texas gulf coast were used to predict the ca mg activity ratios of water in equilibrium with calcite and dolomite to 150 c ca mg activity ratios of these water samples show an excellent linear correlation with temperature giving the equation log aca amg 0 22 7 21t c 1000 r 0 94 n 51 this line parallels and is very near to the equilibrium surface for calcite disordered dolomite suggesting that equilibrium between calcite and poorly ordered dolomite may control the ca mg activity ratios in the water samples in these sediments fe rich dolomite and calcite cements are the last cementation phases loucks et al 1984 indicating they are the most likely minerals affecting the present water chemistry in addition in these sediments calcite cements precipitated in oxygen isotopic equilibrium with present day water and calcite and dolomite cements coprecipitated in oxygen isotopic equilibrium milliken et al 1981 other possible processes such as mixing cation exchange and water rock interaction involving other carbonate and silicate minerals may act as sinks or sources for both ca and mg but they appear not to control the ca mg activity ratios at the study site the ca mg activity ratios of brines from other groundwater systems clastics and dolomite poor carbonates of the u s gulf coast agree well with the ca mg temperature trend determined in this study suggesting equilibrium with calcite and similar composition disordered dolomite in all these sediments water from dolomite rich carbonate sediments did not fit the ca mg activity ratio curve presented in this study and is more likely in equilibrium with calcite and more ordered dolomite a possible effect of dedolomitization this linear ca mg trend was used to estimate the ordering parameter s 0 4 and with that the g f 516 05 kcal mol h f 554 20 kcal mol and a solubility product constant of 10 16 9 at 25 c and 1 bar were calculated for the disordered dolomite found in these sediments this dolomite is likely typical of that found in other clastic and dolomite poor carbonate sediments

palaeoecology and plant succession in a borehole through the rhynie cherts lower old red sandstone scotland a cored borehole through the early devonian rhynie cherts at rhynie aberdeenshire ne scotland has revealed 53 chert beds in 35 41 m of core the cherts originated as sinters deposited by hot spring activity chert comprises 4 20 m of the cored succession with the thickest bed representing a single silicification event being 0 31 m thick and the thickest composite chert comprising six beds 0 76 m thick average chert bed thickness is 80 mm forty five plant bearing chert beds are interbedded with sandstones mudstones and shales the sediments were deposited on an alluvial plain with local lakes the area being periodically affected by hot spring activity plants initially colonized both subaerial sand and sinter surfaces rhynia gwynne vaughanii and horneophyton lignieri commonly form the basal parts of the profiles with subsequent colonization by other genera rhynia is commonly found in life position above originally sandy substrates and horneophyton above sinter surfaces the composition of the rhynie vegetation is compared with coeval assemblages and on the basis of current knowledge it is concluded that there is no unequivocal evidence that the plants were adapted to life in the stressed environments in the immediate vicinity of hot springs

jurassic hot spring deposits of the deseado massif patagonia argentina characteristics and controls on regional distribution abstract the deseado massif santa cruz province argentinean patagonia hosts numerous middle to late jurassic age geothermal and epithermal features represented by siliceous and calcareous chemical precipitates from hot springs sinters and travertines respectively hydrothermal breccias quartz veins and widespread hydrothermal silicification they indicate pauses in explosive volcanic activity marking the final stages in the evolution of an extensive jurassic ca 178 151 ma volcanic complex set in a diffuse extensional back arc setting heralding the opening of the atlantic ocean published paleo hot spring sites for the deseado massif plus additional sites identified during our recent field studies reveal a total of 23 locations five of which were studied in detail to determine their geologic and facies associations they show structural lithologic textural and biotic similarities with miocene to recent hot spring systems from the taupo and coromandel volcanic zones new zealand as well as with modern examples from yellowstone national park u s a these comparisons aid in the definition of facies assemblages for deseado massif deposits proximal middle apron and distal siliceous sinter and travertine terraces and mounds with preservation of many types of stromatolitic fabrics that likely were controlled by formation temperature ph hydrodynamics and fluid compositions locally the mapped hot spring deposits largely occur in association with reworked volcaniclastic lacustrine and or fluvial sediments silicic to intermediate lava domes and hydrothermal mineralization all of which are related to local and regional structural lineaments moreover the numerous geothermal and significant epithermal those with published minable resources deposits of the deseado massif geological province mostly occur in four regional nnw and wnw hydrothermal structural belts northwestern northern central and southern defined here by alignment of five or more hot spring deposits and confirmed as structurally controlled by aeromagnetic data the northern and northwestern belts in particular concentrate most of the geothermal and epithermal occurrences hence jurassic hydrothermal fluid flow was strongly influenced by the most dominant and long active geological boundaries in the region the outer limits of the deseado massif horst itself

nacacl saline formation waters frio formation oligocene south texas usa products of diagenesis abstract saline formation waters from reservoirs in oligocene deltaic clastic sedimentary rocks frio formation south texas are derived from three sources mixtures characterize most samples accounting for the wide range in chemistry low salinity water is derived from cenozoic shales in contrast saline water having variable br cl ratios is allochthonous and is derived from at least two sources in mesozoic strata which underly the cenozoic units saline water in this area regardless of br cl ratio contains variable high concentrations of ca calcium is derived primarily from dissolution of plagioclase as demonstrated by petrography 87 sr 86 sr analyses and chemical modelling the mccl 2 or carpenter function of saline formation waters in this area thus can be derived from water rock interaction and cannot be used to identify the ultimate source of solutes reverse osmosis is an unimportant processes in this area and 18 o and d values converge on rock buffered values of approximately 8 o and 30 o respectively

fluvial response to active extension evidence from 3d seismic data from the frio formation oligo miocene of the texas gulf of mexico coast usa tectonic deformation of the land surface is known to influence the gradient water and sediment discharge and the grain size of modern fluvial systems any change in these variables alters the equilibrium of a fluvial system potentially causing a change in channel morphology 3d seismic data from the tertiary miocene age upper frio formation kelsey field south texas in the us are used to examine changing fluvial channel morphology through time during a period of active growth of a rollover anticline in the hangingwall of a normal fault the vicksburg fault the studied interval varies between 22 and 47 m thick and spans several hundred thousand years it consists of an alternation of fluvial sandstones overbank mudstones and coal seismic extractions show the evolution of sinuous fluvial channels during a phase of growth fault activity prior to growth a single sinuous channel is imaged during growth the fluvial system became decapitated by a developing rollover anticline and a highly sinuous drainage network formed with frequent avulsion events headward propagation of streams and related stream capture increased channel sinuosity was spatially associated with increased avulsion frequency in the area down dip to the east of the rollover anticline more than 10 km from the active fault more than 25 m of relative accommodation developed on the flank of the growing rollover anticline compared with on the crest the increased channel sinuosity is interpreted as reflecting an increase in longitudinal valley slope analogous to observations made in flume experiments and modern river systems the increase in avulsion frequency is attributed to increased aggradation as the rivers adjusted back to equilibrium grade following the increase in slope

syndepositional formation of fe rich clays in tropical shelf sediments san blas archipelago panama solid phase chemistry of modern carbonate siliciclastic marine sediments from the san blas archipelago panama reveals the occurrence of an fe mg authigenic clay facies that is dominated by a 7 1 a ferric clay mineral these fe rich clays are highly susceptible to hcl attack and can comprise a large fraction up to 5 4 wt fe of the sediment some of the clays are clearly forming in the marine environment as they coat grains infill carbonate micropores and replace faecal pellets and carbonate shells the mode of occurrence structure and ferric nature of the clay assemblage identifies the facies as verdine which has odinite a dioctohedral trioctohedral fe3 rich 1 1 clay mineral as its key member the verdine facies in san blas occurs in suboxic sediments where iron reduction can be a prevalent organic carbon oxidation pathway sediment radiocarbon ages show that the san blas verdine clays formed within the last few thousand years and in some cases the clays have been extensively converted into pyrite the verdine clays most likely grew by incorporating terrestrially derived fe and al seawater mg and pore water si derived from biogenic opal dissolution similar clay formation processes may be common in other tropical nearshore environments where sediments are derived from highly weathered lateritic soils and may prove to be an important oceanic elemental sink

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

epithermal sinters of paleozoic age in north queensland australia silica sinters deposited from hot springs have been found associated with volcanic rocks of late devonian to late carboniferous age in north queensland australia these deposits have many features in common with modern hot spring sinters such as those of the taupo volcanic zone in new zealand they occur associated with subaerial volcanic rocks and are locally found with air fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common the associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures the silica veins contain low salinity fluid inclusions trapped at epithermal temperatures and there is evidence of boiling textures preserved in the sinters are identical to those found in modern sinters they include columnar structures similar to bacterial stromatolites described from yellowstone national park as well as striated surfaces apparently resulting from silica deposition on filamentous algae plant fossils including oxroadia gracilis are abundant criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system which may be an important guide in exploration for epithermal precious metal deposits

frio formation of texas gulf coastal plain depositional systems structural framework and hydrocarbon distribution the frio formation oligocene miocene is one of the major tertiary progradational wedges of the texas gulf coastal plain and has yielded nearly 6 billion bbl of oil and 60 tcf of gas the frio and its updip equivalent the catahoula formation consists of deposits of two large fluvial and associated deltaic systems centered in the houston and rio grande embayments structural history in the houston embayment is dominated by syndepositional deformation of underlying jurassic salt mobilization of thick undercompacted prodelta and slope muds characterized the tectonic evolution of the deltaic sequence in the rio grande embayment these two major deltaic depocenters are separated by a vertically stacked strike parallel coastal barrier and strand plain system underlying interbedded and transgressive shelf prodelta and continental slope mudstone sequences provide principal source and sealing facies sparse organic geochemical data regional thermal gradients and distribution patterns of hydrocarbons show that large volumes of oil and gas have likely been generated within and effectively expulsed upward and landward from thermally mature normally to moderately undercompacted sequences of bounding mudstone facies all frio depositional systems contain major geologically defined hydrocarbon producing plays oil and gas field productivity data show a log normal frequency distribution for all but the largest fields per volume productivity production styles and types of hydrocarbons within each of the ten recognized plays reflect available source rock types differing thermal and compaction histories and variable reservoir and trap configurations that characterize each depositional system systematic distribution patterns of both physical and chemical properties of produced hydrocarbons can be related to source facies regional thermal regime and post generation modification by 1 continued maturation thermal cracking and deasphalting 2 migration and attendant chromatographic separatio and 3 bacterial alteration

listric normal faults an illustrated summary movement along listric normal faults is instrumental in formation of several types of structural traps e g rollover anticlines and upthrown fault block closures this type of fault may occur where brittle rocks overlie ductile rocks in an extensional regime the extensional regime may be locally derived within a broader stress regime of another type as evidenced by transtension associated with strike slip movement and arched strata in a compressive setting the flattening of the fault reflects an increase in ductility of the rocks with depth and in some cases deformation of the fault due to compaction or tilting of the upthrown block the dip angle may vary along the strike of the fault in response to changes in throw in cross section a listric fault may consist of relatively short en echelon fault segments this geometry may be particularly characteristic of growth faults sedimentary faults may sole in ductile strata or they may represent the brittle part of a fault flow system fault patterns commonly are characterized by bifurcation some of which may occur near the ends of individual faults comprising a zone although unequivocal recognition of listric normal faults requires unusually extensive outcrop data close subsurface control or high quality seismic data their presence is suggested indirectly by such features as increasing dip either depth toward the controlling fault reverse drag thick progradational sandstone overlying ductile strata and in some cases arcuate fault patterns basins or uplifts listric normal faults form during rifting drifting and evolution of passive continental margins with concomitant basinal development the basement is offset by listric faults as a fundamental element in the development of other types of basins including those which formed during postorogenic extension they also occur as secondary extensional features in an overall compressive stress regime due to plate convergence and during transform or strike slip faulting modified journal abstract

evolution of salt structures east texas diapir province part 2 patterns and rates of halokinesis the effects of salt mobilization on aptian and younger post 112 ma strata in the east texas basin can be used to illustrate patterns of dome growth through time and to estimate rates and amounts of salt movement pre aptian domes grew in three areas around the margin of the diapir province apparently in pre aptian depocenters maximum dome growth along the basin axis coincided with maximum regional sedimentation there during the middle cretaceous aptian albian and cenomanian in the late cretaceous the sites of maximum diapirism migrated to the periphery of the diapir province diapirism began after pillows were erosionally breached leading to salt extrusion and formation of peripheral sinks the duration of pillow and diapir stages of growth was subequal ranging from 10 to 30 ma post diapiric stage of growth continued for more than 112 ma in some cases diapirs grew fastest in the early cretaceous when peak growth rates ranged from 150 to 530 m ma 490 to 1 740 ft ma declining in the early tertiary to 10 to 60 m ma 30 to 200 ft ma assuming steady state conditions over periods of 1 to 17 ma strain rates for the rise of the east texas diapirs averaged 6 7 10 16 sec peak gross rate of growth averaged 2 3 10 15 sec similar to slow orogenic rates the evolution of east texas salt domes essentially ended in the early tertiary with uplift rates less than 30 m ma 100 ft ma

hydrogeology of sedimentary basins abstract hydrogeologic environments in sedimentary basins are as variable as are the different types of basins important hydrologic characteristics can be used to distinguish the different types of basin 1 the topographic setting as determined by the geologic and structural history of the basin 2 permeability distribution within the basin and 3 potential energy distributions and flow mechanisms these parameters control residence times of waters rates and directions of saline groundwater flow and the origin and chemical composition of the saline waters the gulf coast and palo duro basins texas exemplify two end member types of sedimentary basins the gulf coast basin is a relatively young tertiary age basin which is presently compacting fluid movement is from the overpressured undercompacted sediments up the structural dip or up fault zones into the hydrostatic section natural fluid pressures are either hydrostatic or overpressured the palo duro is an older paleozoic age basin that has been tectonically uplifted fluid flow is gravity driven from topographically high recharge areas to discharge in topographically low areas fluid pressures are subhydrostatic fluids discharge more easily than they are recharged not all flow is derived by a simple recharge discharge model brines may flow from other basins into the palo duro basin and waters may discharge from the palo duro basin into other basins areal differences in the chemical composition of the basin brines may be the result of different origins

quantification of brittle deformation in burial compaction frio and mount simon formation sandstones abstract brittle deformation together with grain rearrangement and ductile grain deformation is a key mechanism of compaction in sandstones quartz cementation by virtue of its impact on sandstone mechanical properties is expected to affect the compaction progress by these various mechanisms sandstone samples of different ages and compositions taken from two basins with contrasting burial histories are used to quantify the relationship between brittle deformation and quartz cementation in the context of burial compaction exponential trends of increasing deformation by microfracturing are observed in both the lithic rich frio formation from the gulf of mexico basin r2 0 81 and in the quartz rich mount simon formation from the illinois basin r2 0 78 the two formations contrast in terms of the observed rate of grain fracture increase with depth a larger number of quartz grains in the mount simon formation undergo fracturing at shallow burial 3 km a larger number of quartz grains are fractured in the frio sandstones cathodoluminescence cl imaging conducted on an sem shows that grain fracturing and subsequent cementation of fractures by quartz can occur before during and after normal quartz overgrowth precipitation although fracturing is generally concurrent with cementation the intensity of brittle deformation for a single fractured grain is independent of depth as illustrated by plotting the ratio of the number of particles in fractured grains versus depth r2 0 21 for frio and 0 03 for mount simon suggesting that once fractured grains do not tend to become the locus of subsequent fracturing apparent fracture apertures in the frio grains are slightly wider average 5 m than in mount simon grains average measurable aperture width 4 m quartz grains in the frio have a variety of fracture morphologies including wedge shaped apertures intense comminution at grain contacts and grains with exploded fabrics apparent fracture apertures in mount simon grains are thinner and transect grains as straight uniform traces differences in fracture intensity and pattern are attributed to the fact that cementation in the frio began at greater depth than cementation in the mount simon combining information on the degree of brittle deformation and the amount of quartz cement localized within microfractures here referred to as cf or volume of intragranular cement allows calculation of the amount that brittle deformation influences compaction in the frio formation 0 12 to 8 37 of porosity loss due to compaction can be attributed to brittle deformation whereas the values for the mount simon formation lie between 0 25 to 2 16 the larger values within both formations are affiliated with deeper samples in which a majority of grains manifest fracturing

hydrogen isotope fractionation factor for mixed layer illite smectite at 60 to 150 c new data from the northeast texas gulf coast abstract the d values of water and clay collected from 0 6 to 5 5 km deep wells in 9 oil and gas fields within the houston salt basin located in the northeast texas gulf coast were used to reevaluate the temperature dependence of hydrogen isotope fractionation between mixed layer illite smectite i s and water and the equations of yeh 1980 geochim cosmochim acta 42 140 143 and capuano 1992 geochim cosmochim acta 56 2547 2554 for the calculation of i s water both of which are commonly cited in the literature d values of water and clay in the normally pressured and geopressured sections are different in the normally pressured section