berthierine chamosite corrensite and discrete chlorite from evolved verdine and evaporite associated facies in the jurassic sundance formation wyoming late jurassic sandstones of the shallow marine sundance formation contain authigenic chlorite minerals that occur as rosette like pore fillings of interstratified berthierine chamosite b c and honeycomb like pore linings of corrensite and discrete chlorite b c is nearly ubiquitous in sundance sandstones but is absent near the top of the formation whereas corrensite and discrete chlorite were detected only in uppermost sundance sandstones within 4 m of the contact with the overlying non marine morrison formation glauconite grains are common and occur as laminae along bedding planes and cross beds indicating reworking and deposition as clasts in tidally influenced regimes the mineralogical chemical and morphological properties of the b c and corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals odinite in the case of b c and saponite in the case of corrensite and discrete chlorite odinite has been recognized in numerous shallow marine sands of the holocene verdine facies and the shallow marine conditions associated with sundance deposition would have been ideal for odinite formation saponite commonly forms in aeolian and evaporitic environments implying that the saponite precursor to corrensite and discrete chlorite formed in uppermost sundance sands that were exposed to an influx of oxidizing groundwater following regression of the sundance sea x ray diffraction xrd indicates that the proportion of 7 a b layers in b c ranges from 5 to 28 and chemical analysis by scanning electron microscope energy dispersive x ray spectrometry sem eds indicates positive correlation between b and fe fe mg the polytype of the b c is ibb and the corrensite and discrete chlorite are disordered iib these are the expected polytypes and b for sandstones exposed to burial diagenetic conditions of 3000 4000 m and temperatures 90 120 c

preseismic hydrogen gas anomalies caused by stress corrosion process preceding earthquakes simultaneous monitoring of he h2 ar n2 ch4 and rn in gas bubbles of groundwater from the 1 500 m deep nagashima well in the active yoro fault zone central japan reveals that three microearthquakes with m 1 6 2 6 in the fault zone are accompanied and preceded by increases of h2 as is a moderately large event with m 5 4 in the same fault zone all the focal distances of the earthquakes are within 25 km no comparable anomalies were seen in the gas species other than h2 we suggest that the selective and preseimic increases of h2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress corrosion mechanism aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced h2 production by mechanochemical reaction of fractured rock surface with water in pre existing fault planes as groundwater conduits

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

effects of change in slab geometry on the mantle flow and slab fabric in southern peru the effects of complex slab geometries on the surrounding mantle flow field are still poorly understood here we combine shear wave velocity structure with rayleigh wave phase anisotropy to examine these effects in southern peru where the slab changes its geometry from steep to flat to the south where the slab subducts steeply we find trench parallel anisotropy beneath the active volcanic arc that we attribute to the mantle wedge and or upper portions of the subducting plate farther north beneath the easternmost corner of the flat slab we observe a pronounced low velocity anomaly this anomaly is caused either by the presence of volatiles and or flux melting that could result from southward directed volatile rich subslab mantle flow or by increased temperature and or decompression melting due to small scale vertical flow we also find evidence for mantle flow through the tear north of the subducting nazca ridge finally we observe anisotropy patterns associated with the fast velocity anomalies that reveal along strike variations in the slab s internal deformation the change in slab geometry from steep to flat contorts the subducting plate south of the nazca ridge causing an alteration of the slab petrofabric in contrast the torn slab to the north still preserves the primary fossilized petrofabric first established shortly after plate formation

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

chlorite and biotite weathering fe2 rich corrensite formation and fe behavior under low po2 conditions and their implication for precambrian weathering fresh and weathered granite from drill cores in tono gifu japan was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low p o 2 conditions a fresh sample from 365 m depth a slightly weathered light green sample from 367 m depth and a nearly fresh sample from 369 m depth but with brown stains on fractures were investigated the xrd sem empa and tem analysis of green grains present within chlorite biotite and plagioclase grains and in veins was found to be fe 2 rich corrensite about 40 wt feo with fe fe mg 0 94 the corrensite is interpreted to have formed from chlorite and biotite via a dissolution precipitation mechanism the 2 fe value of 0 69 which when combined with the presence of amorphous fe 3 hydr oxides confirmed by tem indicates that the fe 2 fe value of corrensite is 0 69 these results indicate that on dissolution of chlorite and biotite fe 2 was transported as fe 2 and precipitated as fe 2 rich corrensite and a part of the dissolved fe 2 was oxidized to amorphous fe 3 hydr oxides under low p o 2 conditions the formation of fe 2 rich corrensite and that of fe 2 rich smectite or vermiculite in the laboratory at 1 atm of p co 2 and 3 10 5 atm of p o 2 murakami et al 2004 suggest that a possible fe 2 bearing product during precambrian weathering is fe 2 rich sheet silicates but not siderite

plio quaternary vertical motions and the subduction of the nazca ridge central coast of peru abstract the coastal region of south central peru displays raised marine terraces indicating that unlike the central peruvian coast it underwent a strong uplift since the late pliocene these recent vertical movements which were probably the most rapid motion experienced along the pacific coast of south america are commonly considered to be closely related to the subduction of the aseismic nazca ridge however several questions regarding the geometry and the precise mechanism s responsible for this deformation have not yet been fully addressed the kinematic constraints of the andean convergence suggest that the deformation of the coastal area between pisco and lomas is characterized by a quite continuous sequence of uplift events progressing southwards in spite of some difficulties encountered in precisely dating the uplifted quaternary terraces and other remnants of pliocene marine platforms it is proposed that uplift rates during the late pleistocene reached maximum values of the order of 700 mm 10 3 y as against a recently proposed estimate of 470 mm 10 3 y the reconstructed finite deformation as recorded by the study area for the last 3 ma presents a nw se oriented asymmetric dome shaped pattern it is emphasized that the strongest uplift has been occurring immediately south of the inland projection of the nazca ridge and not along the ridge axis as envisioned by the previously constructed physical models

authigenic chlorites in sandstones as indicators of high temperature diagenesis arkoma foreland basin usa abstract abundant authigenic chamosite associated with higher than average porosities is present in a deep gas reservoir spiro sandstone of the arkoma basin east central oklahoma three types of chlorite can be distinguished petrographically all of which appear texturally to have formed early chlorite peloids diffuse matrix and grain coatings x ray diffraction scanning electron microscopy and electron microprobe studies show that most chlorites are composed of two distinct polytypes ib s 90 and iib a third structure ia polytype was identified in only one sample the relative percentage of the high temperature iib structure increases gradually with increasing thermal maturity from 40 at 3 5 r sub o iib chlorite forms rather thick blocky crystals distinct from the thin pseudohexagonal plates typical of low temperature ib chlorite temperature estimates based on data on vitrinite reflectance and fluid inclusions suggest that iib chlorite formed at burial temperatures 150 180 c higher contents of tetrahedral al3 and slightly higher fe fe mg ratios in iib chlorite are consistent with precipitation temperatures higher than those of the ib structure a higher temperature origin for the iib structure is also consistent with oxygen isotope data

source characteristics of the 12 november 1996 m w 7 7 peru subduction zone earthquake the 12 november 1996 m w 7 7 peru subduction zone earthquake occurred off the coast of southern peru near the intersection of the south american trench and the highest topographical point of the subducting nazca ridge we model the broadband teleseismic p waveforms from stations in the global seismic network to constrain the source characteristics of this subduction zone earthquake we have analyzed the vertical component p waves for this earthquake to constrain the depth source complexity seismic moment and rupture characteristics the seismic moment determined from the nondiffracted p waves is 3 5 1020n m corresponding to a moment magnitude m w of 7 6 7 7 the source time function for the 1996 peru event has three pulses of seismic moment release with a total duration of approximately 45 50 seconds the largest moment release occurs at approximately 35 40 seconds and is located 90km southeast of the rupture initiation approximately 70 of the seismic moment was released in the third pulse

fault plane solutions of earthquakes and active tectonics of the tibetan plateau and its margins summary fault plane solutions of earthquakes within and on the margins of the tibetan plateau show diverse styles of faulting and deformation with thrust faulting and crustal shortening normal to the margins of the plateau and with normal and strike slip faulting resulting in roughly east west crustal extension within the plateau the direction of overthrusting of the himalaya onto the indian shield is radially outward varying from southwest in the western himalaya to south southeast in the east assuming that the indian shield behaves rigidly this requires a west northwest divergence of western tibet from southeastern tibet at a rate of 18 9 mm yr 1 comparable with the rate of convergence at the himalaya fault plane solutions of earthquakes in the southern portion of the tibet plateau consistently show large components of normal faulting on roughly north striking planes and corroborate such extension within the high plateau where elevations exceed 5000 m normal and strike slip faulting occur so that an overall east southeast west northwest extension of the region at about 10 mm yr 1 is partitioned into roughly equal parts of crustal thinning and north northeast south southwest crustal shortening about 5 mm yr 1 in general strike slip faulting characterizes solutions for earthquakes within eastern tibet where mean elevations drop below 4500 5000 m but the orientations of the strike slip faults vary across the region in central tibet left lateral slip occurs on planes trending roughly northeast but for earthquakes farther east the orientations of that plane become progressively east west and then southeast this variation in orientation implies a rotation of material along curved left lateral shear zones thus the eastward extrusion of tibet appears to be facilitated not only by rapid left lateral shear but also by large clockwise rotations of the material in eastern tibet the rate of eastward extrusion of material in eastern tibet relative to the tarim basin to its north is roughly 30 40 mm yr 1 fault plane solutions of earthquakes in the northern and eastern margins of tibet show large components of thrust faulting with the p axes oriented radially outward from the plateau and approximately perpendicular to the regional topographic contours of the plateau the orientation of this crustal shortening is northeast southwest on the northeastern margin east west on the eastern margin and northwest southeast in the longmenshan on the southeastern margin thus at least some of the extrusion of eastern tibet out of india s northward path into asia is absorbed by crustal shortening on the margins of the plateau the variation from normal faulting in the high tibetan plateau where elevations exceed 5000 m to dominantly strike slip faulting farther east where elevations are lower and then to thrust faulting on the margins of the plateau where elevations drop below 3000 m surely results at least in part from a decrease in the value of the vertical stress the magnitude of the east west compressive stress need not vary across the plateau

chloritization of late ordovician k bentonites from the northern baltic palaeobasin influence from source material or diagenetic environment abstract the baltic ordovician silurian sedimentary succession embodies numerous altered volcanic ash beds that are illite smectite dominated only a few beds rich in chlorite smectite are known from the upper ordovician pirgu stage of estonia mixed layer chlorite smectite occurs commonly in low grade metamorphic and hydrothermal environments however chloritic k bentonites of the pirgu stage have never been buried deeply and lack signs of metamorphic overprinting in order to understand the origin of chloritization three distinct beds were sampled in 14 drillcores from estonia and latvia and analysed by means of xrd and sem the principal authigenic assemblage of the bulk samples consists of chlorite smectite corrensite together with illite smectite and k feldspar the actual mineral composition of k bentonites though varies from sample to sample the clay mineral assemblages range from virtually pure chlorite smectite to illite smectite dominated assemblages with minor or no chlorite smectite the proportion of chlorite smectite in k bentonites shows systematic lateral variations the share of chloritic phases is highest in the shallower water part of the palaeobasin and decreases towards the deeper part of the basin such regular lateral variations suggest a possible link between chloritization and the configuration of ancient palaeobasin the present study suggests that the chloritization of primary felsic ashes occurred during early diagenesis and that it was caused by an influx of mg rich water probably from a marine sabkha type environment

early diagenesis of inner shelf phosphorite and iron silicate minerals lower cretaceous of the orpheus graben southeastern canada implications for the origin of chlorite rims wells in the orpheus graben encountered the most proximal part of the deltaic lower cretaceous rocks of the scotian basin more distal sandstones are important gas reservoir rocks with good reservoir quality where fe rich chlorite chamosite rims on framework grains have inhibited quartz cementation cutting samples from the orpheus graben show the presence of fe rich sheet silicates berthierine or chamosite and early diagenetic phosphorite these minerals were analyzed by electron microprobe and their textures were mapped with backscattered electron images studies in the north sea have shown a relationship between high phosphorus and the presence of good chlorite rims in reservoir rocks the mineralization of pore water phosphorus instead of its return to seawater is favored by fe rich sediments and sorption on iron oxides during shallow sea floor diagenesis the fe ti and p contents are uncommonly high in scotian basin shales compared with global average shale compositions the uncommon occurrence of inner shelf phosphorite in this study is interpreted to be a consequence of the same high fe content of the sediment that also favors the formation of fe rich sheet silicates in rapidly deposited deltaic sandstones of the offshore reservoirs the dominance of type 3 kerogen led to sulfate depletion occurring at depths of tens of meters and a corresponding great thickness for overlying eh oxidation potential controlled diagenetic zones the thick fe reduction zone allowed the formation of early diagenetic berthierine which on burial formed the chamosite rims that resulted in the improved reservoir quality in sandstones the distribution of phosphorus minerals may be an indicator of conditions suitable for berthierine formation

mechanoradical h2 generation during simulated faulting implications for an earthquake driven subsurface biosphere 1 molecular hydrogen h2 is the key component to link the inorganic lithosphere with the subsurface biosphere geochemical and microbiological characterizations of natural hydrothermal fields strongly suggested that h2 is an important energy source in subsurface microbial ecosystems because of its metabolic versatility one of the possible sources of h2 has been considered as earthquakes mechanoradical reactions on fault surfaces generate h2 during earthquake faulting however it is unclear whether faulting can generate abundant h2 to sustain subsurface chemolithoautotrophic microorganisms such as methanogens here we present the result of high velocity friction experiments aimed to estimate the amount of h2 generated during earthquakes our results show that h2 generation increases with frictional work i e earthquake magnitude and that a h2 concentration of more than 1 1 mol kg of fluid can be achieved in a fault zone after earthquakes of even small magnitudes the estimated earthquake derived h2 concentration is sufficiently high to sustain a h2 based subsurface lithoautotrophic microbial ecosystem furthermore earthquakes have initiated on the earth at least since tectonic plate movement began 3 8 ga implying the possible existence of ancient earthquake driven ecosystems seismic h2 based subsurface ecosystems might exist not only over the earth but also other planets

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

monitoring of hydrogen along the san andreas and calaveras faults in central california in 1980 1984 hydrogen h2 has been monitored continuously at 1 5 m depth at nine sites along the san andreas and calaveras faults in central california since december 1980 site characteristic small noninstrumental diurnal variations were recorded during quiescent periods at most sites abrupt h2 changes were observed concurrently at two sites on the calaveras fault some of these were correlated with oscillatory fault slips large 1000 4000 ppm h2 increases were recorded at some sites on the san andreas fault between july 1982 and november 1983 which may be correlated with eleven m 5 earthquakes that occurred near coalinga during this period we attribute both the h2 increases and the triggering of the earthquakes to a large scale compressive stress field within the ductile mafic crust near the plate boundary the stress perhaps caused bulging of the base of the brittle upper crust and thus caused dilation of the san andreas fault zone allowing the escape of pent up h2 generated by hydration reaction of the mafic crust at the same time mobile serpentinites may have squeezed into the seismogenic fault beneath the coalinga area triggering the earthquakes

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

prediction of deep reservoir quality using early diagenetic process models in the jurassic norphlet formation gulf of mexico we have developed process based models for early grain coats and their impact on deep reservoir quality in the jurassic eolian norphlet formation alabama with implications for exploration and development in other conventional and tight gas continental reservoirs the norphlet a major gas reservoir to depths of 21 800 ft 6645 m and temperatures of 419f 215c displays contrasting intervals of high and low reservoir quality within compositionally similar cross bedded eolian sands study results show that grain coats formed soon after deposition are responsible for differences in deep norphlet porosity of up to 20 and permeability up to 200 md three types of grain coats were identified in norphlet dune sands each formed in a different part of a shallow groundwater system and each with distinctive impact on deep reservoir quality diagenetic chlorite coats formed where dunes subsided into shallow hypersaline groundwater preserve good deep porosity to 20 and permeability to 200 md continuous tangential illitic coats formed in the vadose zone of stabilized dunes exposed to periodic fresh water influx preserve good deep porosity to 15 associated with poor permeability 1 md due to linked formation of later high temperature diagenetic illite discontinuous grain coats formed in active dunes where grains were abraded by eolian transport are associated at depth with tight zones of pervasive quartz cement low porosity 8 and low permeability 1 md these concepts plus data from 60 wells were used to derive bay wide predictive tight and porous zone isopachs that can be used for well placement geologic models and field development

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

crustal deformation and stress transfer during a propagating earthquake sequence the 2013 cook strait sequence central new zealand the 2013 cook strait earthquake sequence began on 18 july 2013 with two foreshocks of mw 5 7 and mw 5 8 and culminated in the mw 6 6 cook strait and lake grassmere events on 21 july and 16 august respectively located 50 km south of new zealand s capital wellington the earthquakes generated the most significant ground shaking in the wellington and marlborough regions in recent decades during the first event located under cook strait continuously recording gps instruments across central new zealand recorded up to 5 cm of horizontal displacement modeling suggests that the rupture was 25 km long with up to 90 cm of dextral strike slip the second event located 20 km to the southwest caused displacements of up to 25 cm at gps sites located around the clifford bay area in addition two interferograms from radarsat 2 and terrasar x showed up to 30 cm of line of sight displacement in the vicinity of lake grassmere modeling indicates predominantly dextral strike slip of up to 2 1 m coulomb stress changes induced by the earlier foreshocks suggest that the cook strait event was triggered by the preceding events and that the lake grassmere event was subsequently triggered by the cook strait earthquake

depositional controls on glaucony texture and composition upper jurassic west siberian basin this study examines textural inhomogeneity and variable chemical composition of upper jurassic glaucony in relation to small scale synsedimentary and postsedimentary authigenic processes controlled by the palaeonvironmental and palaeogeographical context four glaucony types with complex textural and compositional features have been recognized in cores of the georgiev formation of the west siberian basin samples exclusively made of light green type 1 glaucony k2o 6 5 the less mature type richer in glauconite smectite mixed layer formed under dysoxic conditions in the deepest distal marine environments of the northern sectors of the west siberian basin dark green type 2 glaucony is the most mature richest in glauconitic mica k2o up to 8 5 is sometimes associated with type 1 glaucony and is typical of high bottom areas with a low sedimentation rate within the central sectors of the basin type 3 glaucony is formed by brown grains poorer in k and fe but richer in al and si than type 2 glaucony and is only present in strongly condensed successions of the central eastern sectors of the west siberian basin type 4 glaucony is much richer in fe than any other type shows fresh yellowish green cores slightly less mature than type 2 glaucony and brown rims and cracks with composition similar to that of type 3 grains it was formed in western sectors of the west siberian basin close to urals weathering under a subtropical to temperate climate and erosion of badly drained peneplaned lowland areas around the basin provided al rich terrigenous clays as substratum for glauconitization which explains al and si enrichment in siberian glaucony maturation from glauconite smectite to glauconitic mica is monitored by a change from light to dark green colour related to decrease in al si mg ca and na and to increase in k and fe brown rims of type 4 glaucony and brown type 3 grains formed after leaching of fe and k from mature glauconite with formation of clays and fe oxyhydroxides as reaction products as a result of free oxygen exposure related to a hydrodynamic regime and temporary sea level fall glauconitization stopped and diagenetic pyrite formed due to basin deepening and burial under black shales during the latest jurassic earliest cretaceous transgression this study demonstrates that due to the complex nature of glaucony the authigenesis of glauconitic minerals in the rock record cannot be correctly understood if the palaeoenvironmental context and the palaeogeographical context of glaucony bearing sediments are not considered

cretaceous to early paleogene tectonic evolution of the northern central andes 0 18 s and its relations to geodynamics the tectonic phases subsidence patterns and location of the magmatic arc along the peruvian margin during late cretaceous and paleogene times are compared with the main geodynamic parameters absolute plate motion convergence velocity and direction age of the subducted slab and accretional events for the same periods during the late cretaceous long termed compression seems to be controlled by the absolute trenchward motion of the overriding plate and to a minor extent by the young age of the subducted lithosphere short lived contractional or extensional phases are mainly linked to the acceleration or deceleration of the convergence between the oceanic and continental plates and probably to changes in the convergence direction periods of high and low convergence rates coincide with increased and decreased subsidence rates along the margin respectively and seem to be independent of compressional tensional regimes during the paleogene in addition to the mentioned processes the eastward shift of the magmatic belt and subsidence in the forearc zone are interpreted as the result of the subduction erosion of the margin this process seems to be related to the decrease of the dip of the benioff zone expressed by the drastic widening of the magmatic arc

hydroquakes central apennines italy towards a hydrogeochemical monitoring network for seismic precursors and the hydro seismo sensitivity of boron abstract hydrogeochemical changes before mw 5 0 earthquakes are hoped to be applicable as seismic precursors it is now necessary to pass from single experiences to structured networks systematically monitoring large seismic areas for long periods our aim is to provide a pilot methodological experience for how such networks could be realized and the types of results they could deliver we present the method and some initial results to document the method s applicability in 2017 we started the hydroquakes project in seismically active central italy with the sulmona and matese areas selected as the two monitoring nodes each node includes 5 6 monitored springs and for each spring we performed laboratory analyses of major and trace elements and some isotopes we installed in situ probes for continuous monitoring between december 2017 and january 2020 the most energetic earthquakes were three events with mw of 3 9 4 1 and 4 4 they were not preceded by significant hydrogeochemical anomalies except for the mw 4 1 event despite its low mw this earthquake occurred over a large normal fault at a great depth in the crust 17 km and was preceded by weak but detectable hydrogeochemical anomalies of boron and 11b that were similar but milder than the hydrogeochemical anomalies before the 2016 mw 6 0 earthquake in the central apennines this and previous evidence show the b and 11b hydro seismo sensitivity and indicates that pre seismic hydrogeochemical anomalies could arise mostly before mw 5 0 earthquakes with weak exceptions occurring in specific conditions such as long and deep faults

oxidation of water to hydrogen peroxide at the rock water interface due to stress activated electric currents in rocks common igneous and high grade metamorphic rocks contain dormant defects which release electronic charge carriers when stressed rocks thereby behave like a battery the charge carriers of interest are defect electrons h e g electronic states associated with o in a matrix of o2 known as positive holes or pholes for short the h travel along stress gradients over distances on the order of meters in the laboratory and kilometers in the field at rock water interfaces the h turn into o radicals e g highly reactive oxygen species which oxidize h2o to h2o2 for every two h charge carriers one h2o2 molecule is formed in the laboratory the battery circuit is closed by running a cu wire from the stressed to the unstressed rock in the field closure of the circuit may be provided through the electrolytical conductivity of water the discovery of h charge carriers their stress activation and their effect on earth s surface environment may help better understand the oxidation of the early earth and the evolution of early life

impact of diagenesis on reservoir quality evolution of the late cenomanian abu roash g member in the sitra field north western desert egypt abstract reservoir quality evolution of the abu roash g member was studied by petrographical analyses from wells in the sitra oilfield in the north western desert of egypt the sandstones of the abu roash g member are predominately subarkose to quartz arenite and exhibit a wide range of porosity 1 2 23 8 and permeability 0 008 126 md diagenetic features that influenced the reservoir quality evolution include mechanical compaction pressure solution cementation and dissolution of framework grains and cements compaction and cementation are generally the main factors that were responsible for the reduction of porosity and permeability in the upper abu roash g sandstones dissolution of detrital feldspars grains was the main drive for porosity enhancement of the upper abu roash g sandstones additionally the corroded contacts between successive carbonate phases and quartz grains followed by dissolution of the carbonate cements left behind secondary porosity on the other hand the main porosity preservation in the upper abu roash g sandstones is due to scattered patches of carbonate cement which prevented compactional collapse of the sandstone framework and to grain coating chlorites that inhibited the precipitation of quartz overgrowths during burial the abu roash g member reservoir was subjected to a high diagenetic overprint resulting in marked reservoir heterogeneity

pre earthquake signals underlying physical processes abstract prior to large earthquakes the earth sends out transient signals sometimes strong more often subtle and fleeting these signals may consist of local magnetic field variations electromagnetic emissions over a wide range of frequencies a variety of atmospheric and ionospheric phenomena great uncertainty exists as to the nature of the processes that could produce such signals both inside the earth s crust and at the surface the absence of a comprehensive physical mechanism has led to a patchwork of explanations which are not internally consistent the recognition that most crustal rocks contain dormant electronic charge carriers in the form of peroxy defects o 3 si oo sio 3 holds the key to a deeper understanding of these pre earthquake signals from a solid state physics perspective when rocks are stressed peroxy links break releasing electronic charge carriers h known as positive holes the positive holes are highly mobile and can flow out of the stressed subvolume the situation is similar to that in a battery the h outflow is possible when the battery circuit closes the h outflow constitutes an electric current which generates magnetic field variations and low frequency em emissions when the positive holes arrive at the earth s surface they lead to ionization of air at the ground air interface under certain conditions corona discharges occur which cause rf emission the upward expansion of ionized air may be the reason for perturbations in the ionosphere recombination of h charge carriers at the surface leads to a spectroscopically distinct non thermal ir emission

chemical signature detection of groundwater and geothermal waters for evidence of crustal deformation along fault zones abstract common crustal igneous and high grade metamorphic rocks contain peroxy defects which remain dormant until the rock experiences stress upon deformation stress activated peroxy defects form highly reactive oxygen species ros at the rock water interface observed in both terrestrial and extraterrestrial rock samples the ros complex is formed upon rock fluid interaction when groundwater combines with the ros to produce hydrogen peroxide giving an indirect measure of the underlying rock stress and crustal deformation along active faults the average concentration of hydrogen peroxide from the sohna thermal spring water falling along the active sohna fault and geothermal field was measured to be 0 53 0 014 mg l similar average concentration was also recorded from borewell sample north of the spring 0 56 0 017 mg l within the same geothermal field lower concentration of hydrogen peroxide was recorded within samples collected from hard rock aquifers outside the geothermal field and the active fault zone 0 138 0 095 mg l the detailed analysis suggests that the presence of hydrogen peroxide is largely dependent on the circulation pattern through the interconnected fractures joints within the quartzite bedrock lying adjacent to the fault plane laboratory analysis results also show high hydrogen peroxide concentration within two borewell samples falling adjacent to the epicentre of a shallow microtremor event within the hard rock region falling alongside a lineament along the delhi hard rock mega fold rock water interaction post stress activation of peroxy defects and formation of ros has therefore been inferred to be responsible for the formation of hydrogen peroxide at the rock water interface within these zones this pilot study provides scientific field evidence that hydrogen peroxide concentrations within groundwaters might offer prospects for seismic stress estimation and crustal deformation along seismically active zones

controls on regional variability in marine pore water diagenesis below the seafloor in upper jurassic lower cretaceous prodeltaic sandstone and shales scotian basin eastern canada abstract diagenesis in the uppermost jurassic to lower cretaceous deltaic sandstones and shales of the scotian basin is an important control on reservoir quality ferruginous zone sub oxic marine pore water diagenesis controls the initial formation of fe2 silicates that are the precursors of grain rimming chlorite that preserves porosity this study assesses the regional controls on the type of marine pore water diagenesis by studying the sedimentology mineralogy and geochemistry of the retrogradational units and underlying progradational units in parasequences from conventional cores in two wells in different parts of the basin coated grains preserve a record of whether marine pore water diagenesis below the seafloor was dominantly in the ferruginous or sulphidic geochemical zone four types of coated grain were distinguished each with a different mineral paragenesis mineralogical and chemical evidence of ferruginous zone diagenesis includes the presence of diagenetic chlorite and siderite and the correlation of p with fe or ti pyrite and fe calcite are found where the sulphidic zone is more significant than the ferruginous zone ferruginous zone diagenesis was common in low sedimentation rate retrogradational sediments with low organic carbon and in delta front turbidites and river mouth sandstones estuarine tidal flat and prodeltaic facies that are directly supplied by riverine sediments have a lower fe ti ratio than do fully marine shoreface and open shelf facies as a result of input of detrital ilmenite and its alteration products the relative contribution of colloidal iron hydr oxides appears greater in distal low sedimentation rate environments where large changes in sedimentation rate occurred at ravinement surfaces the underlying progradational rocks have evidence of ferruginous zone diagenesis whatever their facies rapid upward migration of the pore water profile resulting from the change in sedimentation rate reduced the time available for mineral products to form in the deeper pore water zones this study has shown that the availability of fe and organic carbon varying in a complex manner in marine deltaic sediments but that the resulting diagenesis by marine pore water can be predicted from facies and paleogeographic setting

origin of hydrogen and carbon dioxide in fault gases and its relation to fault activity gases occluded in fracture zones of active faults are characterized by a high concentration of h 2 and or co 2 a predominant gas species h 2 or co 2 is related to the lithofacies which the fault cuts carbon dioxide concentration in sediments fluctuates with temperature this evidence and sigma 13 c of co 2 about 20 suggests that co 2 originates from organic materials carbon dioxide with sigma 13 c of 5 to 17 in brecciated gneiss containing marble may have been produced by interaction between organically derived co 2 and the marble or alternatively may have been magmatically derived hydrogen usually occurs in sheared silicate rocks and its concentration fluctuates a great deal the concentration of h 2 from active faults associated with historical earthquakes usually amounts to as high as several percent in maximum whereas the concentration of h 2 from quaternary faults not associated with historical earthquakes is at most 1

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

electro magnetic earthquake bursts and critical rupture of peroxy bond networks in rocks abstract we propose a mechanism for the low frequency electromagnetic emissions and other electromagnetic and electric phenomena which have been associated with earthquakes the mechanism combines the critical earthquake concept and the concept of crust acting as a charging electric battery under increasing stress the electric charges are released by activation of dormant charge carriers in the oxygen anion sublattice called peroxy bonds or positive hole pairs php where a php represents an o 3 x oo yo 3 with x y si 4 al 3 i e o in a matrix of o 2 of silicates we propose that php are activated by plastic deformations during the slow cooperative build up of stress and the increasingly correlated damage culminating in a large critical earthquake recent laboratory experiments indeed show that stressed rocks form electric batteries which can release their charge when a conducting path closes the equivalent electric circuit we conjecture that the intermittent and erratic occurrences of em signals are a consequence of the progressive build up of the battery charges in the earth crust and of their release when crack networks percolate through the stressed rock volumes providing a conductive pathway for the battery currents to discharge em signals are thus expected close to the rupture either slightly before or after that is when percolation is most favored the proposed mechanism should be relevant for the broader understanding of fractoemissions

evidence for strain rate variation and an elevated transient geothermal gradient during shear zone evolution in the cordillera blanca peru abstract the cordillera blanca shear zone cbsz occupies the footwall of the cordillera blanca detachment fault an active low moderate angle detachment fault extending for 200 km along the western margin of the cordillera blanca in central peru recrystallized quartz paleopiezometry two feldspar thermometry on asymmetric strain induced myrmekite and structural analysis from the cbsz provide evidence for shear zone evolution within a transient elevated geotherm during syn convergent extension shear zone width varies by a factor of six along strike with widths of up to 400 m within the central segment of the shear zone microstructurally derived strain rates calculated from quartz paleopiezometry and two feldspar thermometry results are on the order of 10 14 to 10 12 s 1 and are broadly consistent with macrostructurally derived strain rates of 10 14 to 10 13 s 1 calculated from shear zone thickness and previously determined exhumation rates within the context of existing thermochronological data constraints on batholith emplacement and previously determined offset along the cordillera blanca detachment our data suggest that ductile deformation in the cbsz occurred within a transient elevated geothermal gradient of at least 50 c km and potentially as high as 100 c km which deviated from a 30 c km gradient for an unspecified duration of time we discuss the evolution of the cordillera blanca where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism

upper mantle anisotropy beneath peru from sks splitting constraints on flat slab dynamics and interaction with the nazca ridge a r t i c l e i n f o a b s t r a c t the peruvian flat slab is by far the largest region of flat subduction in the world today but aspects of its structure and dynamics remain poorly understood in particular questions remain over whether the relatively narrow nazca ridge subducting beneath southern peru provides dynamic support for the flat slab or it is just a passive feature we investigate the dynamics and interaction of the nazca ridge and the flat slab system by studying upper mantle seismic anisotropy across southern peru we analyze shear wave splitting of sks ssks and pks phases at 49 stations distributed across the area primarily from the peru lithosphere and slab experiment pulse we observe distinct spatial variations in anisotropic structure along strike most notably a sharp transition from coherent splitting in the north to pervasive null non split arrivals in the south with the transition coinciding with the northern limit of the nazca ridge for both anisotropic domains there is evidence for complex and multi layered anisotropy to the north of the ridge our ks splitting measurements likely reflect trench normal mantle flow beneath the flat slab this signal is then modified by shallower anisotropic layers most likely in the supra slab mantle but also potentially from within the slab to the south the sub slab mantle is similarly anisotropic with a trench oblique fast direction but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab overall the regional anisotropic structure and thus the pattern of deformation appears to be closely tied to the location of the nazca ridge which further suggests that the ridge plays a key role in the mantle dynamics of the peruvian flat slab system

seismic structure in southern peru evidence for a smooth contortion between flat and normal subduction of the nazca plate rapid changes in slab geometry are typically associated with fragmentation of the subducted plate however continuous curvature of the slab is also possible the transition from flat to normal subduction in southern peru is one such geometrical change the morphology of the subducted nazca plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry the presence of a thin ultra slow velocity layer usl atop the horizontal nazca slab is identified and located the lateral extent of this usl is coincident with the margin of the projected linear continuation of the subducting nazca ridge implying a causal relationship wherein increased hydration of the ridge results in the formation of the usl downdip waveform modelling of the 2 d structure in southern peru using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab the seismicity and structural evidence suggests smooth contortion of the nazca plate along the transition from flat to normal subduction the slab is estimated to have experienced 10 per cent strain in the along strike direction across this transition

production of hydrogen peroxide in martian and lunar soils freshly ground basaltic minerals produce sufficient quantities of hydrogen peroxide h2o2 when immersed in solution to explain the oxidizing characteristics of martian soil samples collected by the viking landers h2o2 production occurs as a result of reactions between water and defects at the surface of mechanically pulverized minerals analogous to carcinogenic quartz dusts our results also indicate that dehydroxylation of mineral surfaces results in increased h2o2 production a highly relevant finding for dusts on the airless surface of the moon reactive pulverized minerals are important on planetary bodies where impact processes have generated fine grained basaltic dusts and may pose a significant health risk to astronauts visiting planetary surfaces where impacts have generated such materials

crustal deformation and fault slip during the seismic cycle in the north chile subduction zone from gps and insar observations summary the different phases of the earthquake cycle can produce measurable deformation of the earth s surface this work is aimed at describing the evolution of that deformation in space and time as well as the distribution of causal slip on the fault at depth we have applied gps and synthetic aperture radar sar interferometry insar techniques to northern chile where fast plate convergence rates are associated with large subduction earthquakes and extensive crustal deformation the region of northern chile between 18 s and 23 s is one of the most important seismic gaps in the world with no rupture having occurred since 1877 in 1995 the mw 8 1 antofagasta earthquake ruptured the subduction interface over a length of 180 km in the region immediately to the south of this 450 km long gap the coseismic deformation associated with this event has been documented previously here we use gps position time series for 40 benchmarks measured between 1996 and 2000 and ers sar interferograms for the interval between 1995 and 1999 to map both the post seismic deformation following the 1995 event and the ongoing interseismic deformation in the adjacent gap region in the seismic gap the interseismic velocities of 20 30 mm yr 1 to the east with respect to south america are mapped both the gps and the insar measurements can be modelled with 100 per cent coupling of the thrust interface of the subduction to a depth of 35 km with a transition zone extending down to 55 km depth the slip rate in that zone increases linearly from zero to the plate convergence rate south of the gap the interferometric map shows interseismic deformation superimposed with deformation following the 1995 earthquake and covering the same area as the coseismic deformation some 40 per cent of this deformation is related to seismic activity in the 3 3 yr following the 1995 event in particular slip during a mw 7 1 earthquake in 1998 however most of the signal 60 per cent corresponds to post seismic deformation resulting from widespread aseismic slip in the subduction interface the afterslip appears to have occurred down dip in the transition zone at 35 55 km depth and to have propagated laterally northwards at 25 45 km depth under the mejillones peninsula which is a prominent geomorphological feature at the boundary between the 1877 and 1995 rupture zones we propose a simple slip model for the seismic cycle associated with the antofagasta earthquake where the transition zone alternates between aseismic shear and seismic slip

origin of the bolivian andean orocline a geologic study utilizing landsat and shuttle imaging radar abstract the origin of the major andean orocline in bolivia where the north trending southern andes abruptly change direction by 45 and continue northwestward into peru has been investigated by photogeologic techniques utilizing landsat imagery and shuttle imaging radar sir a in conjunction with published literature and maps the evidence suggests that this orocline might have been formed beginning in the miocene by the pacific coastal region being bent around the resistant brazilian shield as a result of the relative motions between the eastward moving coastal region and the westward moving south american continent the eastward motion of the coast was further increased about 10 m y b p when the opening of the east pacific rise increased the rate of eastward motion of the subducting nazca plate the brazilian shield with its deep roots provided greater resistance to this motion resulting in the decoupling of the andes along a wide left lateral strike slip fault zone extending east west from the altiplano through santa cruz and into the interior indicated left lateral motion across this fault decreases from 180 km on the east side of the andes to 50 or 60 km in the altiplano the difference being taken up in compressing the eastern cordillera and sub andean foredeep from an average width of 350 km in southern bolivia and argentina to an average width of 220 km in northern bolivia and peru the resistance of the shield may also have resulted in counter clockwise rotation of and lateral angular shear in the andes north of the break zone

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

ground water radon anomaly before the kobe earthquake in japan radon concentration in ground water increased for several months before the 1995 southern hyogo prefecture kobe earthquake on 17 january 1995 from late october 1994 the beginning of the observation to the end of december 1994 radon concentration increased about fourfold on 8 january 9 days before the earthquake the radon concentration reached a peak of more than 10 times that at the beginning of the observation before starting to decrease these radon changes are likely to be precursory phenomena of the disastrous earthquake

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

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

diagenetic influence on reservoir quality evolution examples from triassic conglomerates arenites in the edvard grieg field norwegian north sea abstract the utsira high southern norwegian north sea has been a petroleum exploration target since the late 1960s the southwestern part of the utsira high complex haugaland high attracted new interest when in 2007 lundin norway as and partners revus energy asa and rwe dea norge as discovered commercial hydrocarbon reservoirs in triassic to early cretaceous formations edvard grieg field in the present study four siliciclastic cores from the edvard grieg field have been investigated sedimentologically including core logging optical mineralogy x ray powder diffraction xrd and scanning electron microscope sem quantification and characterization of the different mineralogical phases in thin section have been executed to better understand porosity evolution through time intergranular volume igv and cement volume were also compared in order to estimate the most important factors effecting porosity loss within the studied samples the results show that through time the various diagenetic events and phases such as dissolution of unstable grains and precipitation of clay minerals and cements strongly influenced reservoir quality in the edvard grieg field two main lithology groups are identified within the cored intervals conglomerates and sandstones these two groups can further be sub divided into four units 1 sandy conglomerate 2 silty conglomerate 3 interbedded sandstone 4 large scale cross bedded sandstone most of the reservoir quality reduction in the conglomerate matrix can be related to poor sorting carbonate cement and pore filling pore lining authigenic clay minerals better reservoir quality characterizes the sandstone dominated alluvial fan intervals which generally are associated with a greater degree of sorting that led to higher intergranular porosities large scale cross stratified sandstone units interpreted to be of aeolian origin have the highest reservoir quality these deposits are moderately to well sorted with only minor occurrences of patchy carbonate cement and generally low proportions of authigenic minerals however locally abundant kaolinite has been observed filling the pore space and reducing the porosity predominantly by blocking interconnected primary pores

measurement of accessible reactive surface area in a sandstone with application to co2 mineralization measurement of accessible reactive surface area in a sandstone with application to co 2 mineralization gautier landrot 1 jonathan b ajo franklin li yang stefano cabrini and carl i steefel glandrot gmail com jbajo franklin lbl gov lyang lbl gov scabrini lbl gov cisteefel lbl gov lawrence berkeley national laboratory 1 cyclotron rd berkeley ca 94720 8099 usa corresponding author glandrot gmail com tel 66 90 79 77 922 fax 66 29 42 81 06 1 present address department of environmental engineering kasetsart university 10 th floor building 14 50 ngamwongwan road jatujak bangkok 10900 thailand

h2 generation by reaction between h2o and crushed rock an experimental study on h2 degassing from the active fault zone granite and quartz were crushed under moist conditions in the temperature range of 25 270 c in order to obtain the information on the mechanism operative for h2 degassing along active faults it was found that the amounts of h2 gas released by crushing granite and quartz increased with temperature up to around 200 c and then decreased suddenly the results indicate that h2 gas is generated by a chemical reaction between water and such radicals as si and si o formed on the fresh surfaces when the si o si bonds are destroyed by crushing and that the existence of a maximum for amount of h2 gas is due to the decrease of the lifetime of si o radical at the temperature higher than about 200 c the results lend credence to the possibility that h2 gas is generated in the temperature range of 25 270 c along the active fault through the reaction between groundwater and the fresh surfaces of underground rocks formed by the fault movements

seismic zonation of the delhi region for bedrock ground motion an endeavor is made to compute peak ground horizontal accelerations at bedrock level in the delhi region due to the seismogenic sources present around delhi the entire area is divided into six seismogenic sources for which seismic hazard analysis is carried out using the complete and extreme part of the seismicity data maximum likelihood estimates of hazard parameters viz seismic activity rate b value and maximum probable earthquake mmax are made for each zone the return periods and the probabilities of occurrence of various magnitudes for return periods of 50 100 and 1000 years are also computed for each zone the peak ground acceleration pga values for 20 exceedance in 50 years are then computed for the delhi region from each zone the maximum pga value considering all the zones is 0 34 g which is due to the mathura fault zone the seismogenic zones v and vi i e mathura fault zone and the sohna fault zone are observed to be contributing maximum pga values in the delhi region governing the isoacceleration contours computed for the region the seismic zonation map for the pga values at the bedrock level is obtained for the delhi region this can be used directly as input for the microzonation of ground motion at the surface by incorporating the local site conditions

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

a chlorite solid solution geothermometer the los azufres mexico geothermal system chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites in the active geothermal system of los azufres mexico electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition correlation coefficients among chemical constituents were calculated it is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content and there is almost no correlation with the octahedral aluminium and magnesium content a procedure is proposed to select end members and substitution vectors and to give a general formula for these chlorites

tidal inlet sequence sundance formation upper jurassic north central wyoming the sandstones and coquinas of the upper 20 m of the sundance formation are interpreted as a tidal inlet back barrier shoal and sandy tidal flat sequence deposited at the close of marine jurassic sedimentation in north central wyoming the barrier strandline maintained a generally e w trend as it prograded to the north the lateral migration of inter barrier tidal inlets along the regressive shoreline of the late sundance sea caused the coquinas and sandstones of the uppermost sundance formation to be deposited as tabular laterally extensive units tidal bundles sigmoidal reactivation surfaces herringbone cross lamination and abundant mud drapes within the sandstones are evidence of considerable tidal influence during the deposition of the uppermost sundance formation earlier models which attach an offshore environment of deposition to the sequence do not explain the tabular geometries of the sandstone and coquina units and their conformable stratigraphic relationship with the overlying non marine sediments of the morrison formation

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

re evaluation of coquinoid sandstone depositional model upper jurassic of central wyoming and south central montana the most extensive jurassic marine transgression in north america reached its maximum limits during the oxfordian age at this time siliciclastic sediments were being brought into the north american seaway from an uplifted zone to the west within this setting complexes of sand ridges and coquinoid sands layers were deposited coquinoid sandstones appear to fill erosional scours and were interpreted as channel fills re evaluation of these features in the light of recently discovered attributes of modern shelf sediments and processes has produced a revised model of coquinoid sand deposition in this setting coquinoid sandstones which fill channel like scours in the oxfordian upper jurassic rocks of central wyoming and south central montana appear to have formed through the migration of sand waves across the crests of inner shelf sand ridges during periods of storm and tidal flow erosion in the zone of flow reattachment in the troughs between sand waves resulted in the development of shell lags migration of these scour zones as the sand waves advanced resulted in the deposition of sheet like coquinoid sandstone bodies sand waves crossing the ridge crest tended to migrate more slowly and to be overstepped by later sand waves sand wave troughs thus buried have channel like geometries with apparent epsilon bedding

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

hydrothermal clay mineral formation in the uraniferous paleoproterozoic fa formation francevillian basin gabon abstract the spatial distribution of neoformed clay minerals was investigated in the paleoproterozoic fa formation of the francevillian basin south east gabon which hosts high grade u ore deposits associated with the only known occurrence of natural nuclear reactors illite appears as the main clay phase in the lower fluvial unit in the mineralized upper fluvio deltaic tidal unit the clay assemblage is more diversified and commonly characterized by illite chamosite berthierine and chamosite berthierine mixed layers usually in association with alteration products of u bearing minerals the clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of u ore deposits polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid rock ratio according to crystalline structure mineral chemistry thermodynamic modeling and geochronology clay phases seem to be mainly hygrometer rather than exclusively a thermometer and their formation as well as associated dissolution precipitation of u bearing minerals took place between ca 2040 and 2010 ma ago at temperatures of about 240 30 c using previous burial estimates this suggests the operation of hydrothermal processes controlled by an external heat source likely associated with volcanism during deposition of the fd and fe formations hydrothermal activity involving oxidized fluids would have driven u remobilization and ultimately formation of high grade u ore deposits the thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich u endowment

gas geochemistry applied to earthquake prediction an overview terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas especially in ussr china japan and united states in search of premonitory changes that might be useful for earthquake prediction concentrations of radon helium hydrogen mercury carbon dioxide and several other volatiles have been found generally to be anomalously high along active faults suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers these sensitive stations are generally located along active faults especially at intersections or bends of faults or some other structurally weak zones possibly because of tectonic strain concentration at such places however gas concentrations especially those measured at or near the ground surface may also be significantly affected by meteorological hydrological and other nontectonic changes in the environment the nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake related anomalies

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

on the origins of eogenetic chlorite in verdine facies sedimentary rocks from the gabon basin in west africa abstract the verdine facies clay assemblages are regularly encountered in shallow marine sediments of equatorial latitudes where fe of terrigenous origin is readily available there the 7 a fe rich green clay odinite of syn sedimentary origin commonly occurs previous findings suggested that odinite serves as a precursor of diagenetic chlorite whose importance in porosity conservation has been widely acknowledged the investigation of fe rich clays is therefore helpful to constrain the specific conditions and mechanisms which give rise to chlorite growth thus illuminating the history of early burial in this contribution we studied cretaceous sediments from the gabon coast where multiple sedimentary basins came into existence prior during and after the opening of the south atlantic analyzed core is siliciclastic and petroleum bearing stemming from the fluvio lacustrine deltaic environment recovered from depths of 1500 m tvd electron micro beam investigation coupled with x ray diffraction on the clay fraction and spectra fitting revealed odinite interstratified odinite chlorite o c and to a lesser extent illite chlorite smectite i c s and illite smectite occur as grain coatings and pore filling phases with a characteristic honeycomb texture embryonic chlorite emerges at the contact of odinite and o c flakes as a particulate authigenic mineralization formed from aluminosilicate and fe oxyhydroxide debris at sediment water interface odinite becomes intrinsically unstable possibly due to the activity of fe reducing bacteria or decaying organic matter creating a prevalently reducing environment that promotes fe reduction such condition facilitated a two step chlorite growth which first included a partial chloritization of odinite through the in situ solid state transformation of 7 a to 14 a layers then with increasing burial at peak diagenetic conditions 50 c 1 5 km depth odinite and o c both recrystallized to form eogenetic chlorite most likely following the ostwald ripening mechanism

coupling between mineral reactions chemical changes in groundwater and earthquakes in iceland chemical analysis of groundwater samples collected from a borehole at hafralaekur northern iceland from october 2008 to june 2015 revealed 1 a long term decrease in concentration of si and na and 2 an abrupt increase in concentration of na before each of two consecutive m 5 earthquakes which occurred in 2012 and 2013 both 76 km from hafralaekur based on a geochemical major elements and stable isotopes petrological and mineralogical study of drill cuttings taken from an adjacent borehole we are able to show that 1 the long term decrease in concentration of si and na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and 2 the abrupt increase of na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of na into groundwater we attribute decay of the na peaks which followed and coincided with each earthquake to uptake of na along fractured or porous boundaries between labradorite and analcime crystals possible causes of these na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components both could have been triggered by preseismic dilation which was also inferred in a previous study by skelton et al 2014 the mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown

the rupture process of the great 1979 colombia earthquake evidence for the asperity model we have determined the asperity distribution in the segment of the ecuador colombia subduction zone that was ruptured by the 1979 mw 8 2 and 1958 mw 7 7 earthquakes for the 1979 earthquake we are able to document directivity in the p wave source time functions and consequently have spatially located the moment release by combining results from both p wave and surface wave directivity we have defined three regions 1 from 0 to 56 km northeast of the epicenter with low moment release and a small seismic displacement 2 the asperity from 56 to 116 km northeast of the epicenter with most of the moment release and a displacement of 6 m and 3 a small displacement region from 116 to 180 240 km northeast of the epicenter thus the 1979 rupture zone is composed of an asperity 60 km in length bounded by two weak small displacement regions the displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 the year of the previous great earthquake in this segment the 1958 earthquake is characterized by the rupture of a small asperity length scale approximately 25 km in the epicentral region the 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event this detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

cenozoic marine sedimentation in the sechura and pisco basins peru abstract the central and northern peruvian margin consists of a series of 8 paired forearc basins which may be separated into an inner set of shelf basins and a seaward set of slope basins we have examined the cenozoic stratigraphy of the onshore portions of the sechura basin 5 7 s and pisco basin 13 16 s two shelf basins which have accumulated marine sediment discontinuously since the mid to late eocene cenozoic sediments in the pisco basin were deposited during at least three major transgressive cycles each sequence is preserved as a similar vertical progression of facies including coarse nearshore bioclastic conglomerates and sandstones grading upwards into sandy siltstones and mudstones and capped by biogenic deposits including diatomites diatomaceous mudstones dolomitic horizons and phosphate deposits stratigraphic nomenclature for the pisco basin has recently evolved a stratigraphy presented here includes the eocene caballas fm upper eocene los choros fm upper eocene to lowermost oligocene yumaque fm uppermost oligocene to middle miocene chilcatay fm and upper miocene to pliocene pisco fm major hiatuses in the pisco basin span the late cretaceous to middle eocene early to late oligocene middle miocene and late pliocene pleistocene to recent cenozoic sediments of the sechura basin were deposited within at least 4 major transgressive cycles with hiatuses during the paleocene to middle eocene oligocene early to middle miocene and late miocene based on recent biostratigraphic studies sediments enriched in biogenic components accumulated between about 40 36 ma 24 16 ma and 11 3 ma in the pisco basin and between 40 37 ma and 8 5 4 5 ma in the sechura basin in both basins the most diatomaceous sediments are restricted to the late eocene and late miocene through pliocene the temporal distribution of biogenic sediments suggests that high productivity conditions linked to coastal upwelling have occurred episodically since at least the late eocene the occurrence of diatomites and phosphorites is diachronous between the pisco and sechura basins and between the peruvian forearc and other circum pacific monterey formation analogs a reflection of the strong influence of local tectonism on sedimentation patterns the volume of neogene sediments along the peruvian forearc is nearly twice that of the monterey fm despite basin to basin facies diachroneity these deposits very likely contributed to fluctuations of the late miocene carbon co 2 system by acting as large carbon sinks

changes in groundwater chemistry before two consecutive earthquakes in iceland precursor events to earthquakes are rarely reproduced measurement of groundwater chemistry in iceland between 2008 and 2013 reveals distinct changes prior to two consecutive m5 earthquakes

experimental production of clay rims by macrobiotic sediment ingestion and excretion processes using controlled experiments that simulate interlayered sand and mud in marine conditions it is shown that clay rims develop on sand grains during the digestive processes of the polychaete worm arenicola marina the clay size minerals also undergo accelerated weathering and mineral authigenesis synthetic mud crushed slate and clean eolian sand of known composition and texture were horizontally layered in experimental and control tanks natural seawater was added to these tanks along with several a marina fecal casts were collected at regular time intervals and these were analyzed texturally using electron microscopy and mineralogically using x ray diffraction xrd total bioturbation of the sand and mud layering by a marina was achieved after only eight weeks of feeding activity sand grains in the fecal casts developed clay mineral rims a feature absent in the original material and not found in the control tanks thin composite curvilinear structures assumed to be detached clay rims were also produced over a period of many months the synthetic mud proved to be unchanged in the control tank but it was significantly different in the fecal casts from the experimental tank that contained the worm a marina the original minerals were degraded and new authigenic clay minerals formed it is postulated that a combination of low ph and the biologically active microenvironment in the guts of annelid worms may radically accelerate mineral dissolution and clay mineral precipitation processes during digestion it is likely that mucous produced by the worms not only led to the aggregation of the fine grained material into fecal casts but also adhered the fine sediment onto sand grains thus creating clay mineral rims in this case the solid part of the clay rim will be a combination of the primary material crushed slate and the neoformed clay minerals

cause and timing of the thermal over maturation of hydrocarbon source rocks of the ecca group main karoo basin south africa abstract the source rocks of the main karoo basin south africa are believed to have undergone thermal over maturation close to the jurassic aged karoo large igneous province klip dolerite intrusions yet the cause and timing of this source rock exhaustion event likely to have extensively destroyed their shale gas potential remain controversial based on drill core samples from the central main karoo basin we investigated the clay mineralogy of the 2 1 1 and 2 1 phyllosilicate groups and have carried out 40ar 39ar radiometric dating of illite the x ray diffractograms xrd and mineral chemistry reveal that chlorite and illite phases of samples close to the dolerite intrusions are characterized by higher crystallinity with a chemical composition indicative of formation in a metamorphic environment in contrast chlorite and illite distal to any dolerite intrusion show lower crystal growth rates with chemical compositions suggesting formation in a diagenetic environment 40ar 39ar radiometric dating indicates that illite in samples distal to the dolerite intrusions formed between 292 and 245 ma with some ages clustering around 281 ma the oldest ages are consistent with a detrital fraction probably recycled from the erosion of the ash bed at the top of the underlying dwyka group however ages around 281 ma can be linked with estimated depositional age of the ecca group at 280 ma while the youngest ages may reflect late diagenesis to regional metamorphism illite fractions from samples close to dolerite shows formation age between 204 and 170 ma consistent with the time of the klip dolerite sills and dykes intrusions at 180 ma this illustrates that the ages of these illites were partially or totally re set due to recrystallization in a late thermal anomaly induced by the intrusion of dolerites the correlation between the timing of the youngest mineral transformation processes and the klip represents strong evidence suggesting that source rocks of the ecca group experienced a thermal overprinting event 180 ma ago this late thermal anomaly is consistent with previous data illustrating thermal over maturation and metamorphic devolatilization of ecca group shales situated close to the dolerite intrusions

ferruginous seawater facilitates the transformation of glauconite to chamosite an example from the mesoproterozoic xiamaling formation of north china berthierine and chamosite are iron rich clay minerals that share similar chemical compositions berthierine forms at low temperature 25 45 c during early diagenesis and may transfer to chamosite at temperatures of 70 c because the formation of berthierine and chamosite requires significant amount of fe 2 supply their presence in marine sediments is often used as a mineral proxy for ferruginous conditions in porewater recent studies reveal that the precambrian oceans were characterized by pervasive ferruginous water column conditions that may favor the formation of iron rich clay minerals like berthierine and chamosite to evaluate if ferruginous water column conditions in the precambrian ocean played a role on iron rich clay mineral formation we conducted an integrated petrographic mineralogical and geochemical study on the chamosite and glauconite bearing strata of the mesoproterozoic xiamaling formation 1 40 1 35 ga in north china petrographic xrd sem and eds analyses show that the chamosites of the xiamaling formation was transferred from glauconite with berthierine as an intermediate mineral phase during early diagenesis geochemical analyses indicate that a complete transformation from glauconite dominated to chamosite dominated end members samples requires an addition of a large amount of fe 16 9 wt mg 2 4 wt and a small amount of al 1 4 wt but a simultaneous release of si 11 8 wt and k 6 0 wt considering that the glauconite and chamosite bearing strata are devoid of iron rich detrital minerals e g biotite and iron oxides and lack evidence of hydrothermal alteration the required fe 2 for glauconite berthierine chamosite transformation was most likely from fe 2 rich ferruginous seawater which may have promoted glauconite berthierine transformation at the very early diagenetic stage when fe 2 exchange between porewater and seawater was still available this interpretation is consistent with the high fe hr fe t but low fe py fe hr fe al and v al ratios from the hosting strata that support ferruginous depositional environments because most precambrian strata have passed the oil window temperature 50 150 c the preservation of berthierine would be rare and chamosite should be the representative iron rich clay mineral thus the abundance of chamosite in fine grained marine siliciclastic sediments may be used as a mineral indicator of ferruginous water column conditions

sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the upper cape hay formation permian bonaparte basin australia abstract the deep siliciclastic reservoir 3500 m of the upper cape hay formation of the bonaparte basin petrel gas field petrel sub basin permian exhibits wide heterogeneity in porosity 2 26 and permeability 0 001 2500 md to investigate this variability 42 samples were taken from five wells drilled through this formation six facies were identified from core descriptions and microscopic study of the sandstones these facies are typical of a tide dominated estuary and include 1 mud flat 2 sand flat 3 top of tidal sand bar 4 middle of tidal sand bar 5 bottom of tidal sand bar and 6 outer estuary facies the paragenetic sequence comprises the emplacement of early aggregates of ferrous clay mineral precursors mechanical compaction recrystallization of those ferrous clay mineral precursors to fe rich chlorite and crystallization of fe rich chlorite forming coatings around detrital grains chemical compaction development of quartz overgrowth feldspar alteration crystallization of dickite and illite rich illite smectite i s mixed layers and ferrous calcite cementation the middle and top of the tidal bars generally exhibit the highest porosity 10 and permeability values k 1 md feldspar alteration released silica and aluminium into the reservoir promoting the development of dickite and illite rich i s mixed layers which tended to destroy porosity and permeability as calcite cements and quartz overgrowths diagenetic chlorite coatings around detrital grains are restricted to the sand bar facies deposited at the end of the last third order transgressive systems tract of the cape hay formation the formation and conservation of ferrous clay precursors seems to be possible in an estuarine environment where seawater and fresh water are mixed and tidal sand bars are formed these ferrous clay precursors recrystallized to fe rich chlorite coating after mechanical compaction these coatings inhibited quartz cementation and prove to be the key parameter behind good reservoir qualities