trace elements content of galenas and associated minerals in some miocene lead zinc deposits near red sea coast egypt the trace elements content of some m miocene egyptian galenas is given and the method is briefly described the relative enrichment in tl and cd together with the irregular distribution and low ag bi and sb content indicate low temperature conditions of formation the trace content of hemimorphites cerussites wulfenites and other minerals associated with galena shows that they are related to ascending exhalative solutions

direct dating of authigenic k feldspar overgrowths from the kilombero rift of tanzania k feldspars form important detrital and authigenic components in sediments but may be difficult to separate by conventional means for isotope analysis particularly geochronology uv lasers are being increasing applied to in situ analysis since they are strongly absorbed by clear minerals such as k feldspar and can achieve a spatial resolution of better than 20 m for individual ar ar analyses here we demonstrate separation of detrital k feldspar with a mean age of 423 14 ma and authigenic overgrowths with a mean age of 138 5 ma in clastic sediments from the msolwa group of the kilombero rift of tanzania the young age obtained for authigenic k feldspar overgrowth demonstrates that precipitation occurred more than 160 ma after sedimentation during the period of deepest burial and synchronous with the initial extensional phases of the intracontinental anza and sudan rifts as well as the initial stages in the break up and separation of south america and africa

site amplification scattering and intrinsic attenuation in the mississippi embayment from coda waves we estimate site amplification intrinsic q q i and scattering q q s values from three component coda of broadband data recorded by cooperative new madrid sesimic network stations for center frequencies 0 2 0 5 1 0 2 0 and 5 0 hz near 0 5 and 1 0 hz site amplification factors for the transverse component recorded by stations in the mississippi embayment are between 1 7 and 18 compared with stations outside the embayment the amplification factor becomes smaller at frequencies lower than 0 5 hz and higher at those greater than 1 0 hz the radial component shows a similar amplification however the vertical component shows relatively less amplification q i and q s are estimated from coda using an energy flux model for two time windows after the direct s wave and at 0 5 and 1 0 hz center frequencies the two time windows include coda up to and after twice the s wave travel time at each station coda parameters change with the time window and are attributed to a mixture of incoherent coda with coherent s wave reverberations directly after the s wave q i and q s inferred from stations outside of the embayment have higher values than those from stations within the embayment a plane wave model qualitatively explains the lack of coda amplification at high frequencies compared with a low frequency without recourse to anelastic attenuation arguments however the plane wave model cannot explain the very large observed coda amplifications at 1 and 0 5 hz seen in the data suggesting that other wave scattering effects must be occurring in the embayment velocity structure this is consistent with the relatively low q s values inferred from the energy flux model for stations within the embayment in addition to coda amplification we performed a synoptic noise study that revealed that pre event ambient background noise behaves in a similar manner to the coda for stations within and outside of the embayment the presence of large lateral velocity heterogeneities in the embayment may cause q s values to be low there

hydrocarbon composition of authigenic inclusions application to elucidation of petroleum reservoir filling history abstract geochemical analysis of petroleum inclusions trapped in authigenic feldspar and quartz in the ula formation in the north sea ula oil field revealed a petroleum of markedly different composition than the oil presently in the reservoir using microthermometry and the burial history as a dating tools it is concluded that the petroleum in the k feldspar inclusions was present in the more porous and permeable parts of the ula formation as early as 45 75 my bp when the field was at a depth of about 1 0 1 5 km as compared with the current depth of 3 4 km this early petroleum which was trapped as inclusions in authigenic k feldspar shows a distinctly different distribution of tricyclic terpanes and pentacyclic triterpanes from that of the current petroleum charge in the ula formation which was derived from the mandal formation source rock in late neogene time molecular parameters show that the oil in the k feldspar inclusions is significantly less mature than the crude oil in the present reservoir the approximate 90 c temperature increase occurring after entrapment of the early petroleum in kfeldspar the field is currently at 143 c appears not to have reset the low maturity signature of the oil in the k feldspar inclusions this could suggest that the temperature in the inclusions is too low for isomerization selective thermal degradation to occur lack of catalysts or that there are other controls on the ratio of some of these parameters still parameters like the ratio of c21 to c28 triaromatic steroids and those based on dimethyl and trimethyl naphthalenes are comparatively similar in both the inclusions and in the reservoir crude the oil inclusions in authigenic quartz and albite formed from about 10 my bp burial depth 2 5 km until the present burial depth 3 4 km are interpreted as representing a palaeo petroleum charge having a composition intermediate between the oil found in k feldspar inclusions and the oil charge in the present reservoir our data suggest strongly that the petroleum trapped in the inclusions in quartz and plagioclase represents progressive dilution of the early reservoired oil now found only in k feldspar inclusions with oil from the mandal formation analysis of solution gas in these inclusions suggests that the first petroleum associated gas to arrive at the trap from the progressively maturing mandal formation had a much wetter composition than the current reservoir solution gas to explain the present comparatively drier and isotopically lighter reservoir solution gas composition the inference is made that there has been a recent during formation of k feldspar inclusions 45 75 my bp maturation of the upper jurassic mandal formation the main source rock in this region was limited possibly to the graben axis this suggests that the petroleum found in the k feldspar inclusions was most likely derived from a deeper post caledonian palaeozoic formation

origin and morphology of limestone caves limestone caves form along ground water paths of greatest discharge and solutional aggressiveness flow routes that acquire increasing discharge accelerate in growth while others languish with negligible growth as discharge increases a maximum rate of wall retreat is approached typically about 0 01 0 1 cm yr determined by chemical kinetics but nearly unaffected by further increase in discharge the time required to reach the maximum rate is nearly independent of kinetics and varies directly with flow distance and temperature and inversely with initial fracture width discharge gradient and pco2 most caves require 104 105 yr to reach traversable size their patterns depend on the mode of ground water recharge sinkhole recharge forms branching caves with tributaries that join downstream as higher order passages maze caves form where 1 steep gradients and great undersaturation allow many alternate paths to enlarge at similar rates or 2 discharge or renewal of undersaturation is uniform along many alternate routes flood water can form angular networks in fractured rock anastomotic mazes along low angle partings or sponge work where intergranular pores are dominant diffuse recharge also forms networks and spongework often aided by mixing of chemically different waters ramiform caves with sequential outward branches are formed mainly by rising thermal or h2s rich water dissolution rates in cooling water increase with discharge co2 content temperature and thermal gradient but only at thermal gradients of more than 0 01 c m can normal ground water co2 form caves without the aid of hypogenic acids or mixing artesian flow has no inherent tendency to form maze caves geologic structure and stratigraphy influence cave orientation and extent but alone they do not determine branch work versus maze character

evolution of fault zones in carbonates with mechanical stratigraphy insights from scale models using layered cohesive powder abstract we present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones using cohesive hemihydrate powder caso 4 h 2 o the evolution of these dilatant fault zones involves a range of processes such as fragmentation gravity driven breccia transport and the formation of dilatant jogs to allow scaling to natural prototypes extensive material characterisation was done this showed that tensile strength and cohesion depend on the state of compaction whereas the friction angle remains approximately constant in our models tensile strength of the hemihydrate increases with depth from 9 to 50 pa while cohesion increases from 40 to 250 pa we studied homogeneous and layered material sequences using sand as a relatively weak layer and hemihydrate graphite mixtures as a slightly stronger layer deformation was analyzed by time lapse photography and particle image velocimetry piv to calculate the evolution of the displacement field with piv the initial predominantly elastic deformation and progressive localization of deformation are observed in detail we observed near vertical opening mode fractures near the surface with increasing depth dilational shear faults were dominant with releasing jogs forming at fault dip variations a transition to non dilatant shear faults was observed near the bottom of the model in models with mechanical stratigraphy fault zones are more complex the inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation

fissure fills along faults variscan examples from gower south wales the extent to which persistent rather than transient fissures wide planar voids can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon bearing fluids variscan late carboniferous faults cutting dinantian lower carboniferous limestones on the gower peninsula south wales host clear evidence for fissures up to several metres wide evidence includes dendritic hematite growth and elongate calcite growth into open voids spar ball and cockade breccia formation laminated sediment infill and void collapse breccias detailed mapping reveals cross cutting geometries and brecciation of earlier fissure fills showing that fissures were formed during rather than after active faulting fissures therefore probably formed by geometric mismatch between displaced fault walls rather than by solution widening along inactive faults

shear zones as a new type of palaeostress indicator this paper proposes that shear zones act as palaeostress indicators in much the same way as brittle faults several hundred shear zones from both the limbs of the variscan langland mumblcs anticline south wales are treated using the graphical and numerical methods of fault slip analysis estimated palaeostress directions derived from shear zones are characterized by greater scatter than where brittle faults are used as stress indicators an important reason for this is the fact that on some shear zones the direction of maximum shear stress slip lineation is not perpendicular to the line of s c intersection as a consequence the use of slip lineations produces better solutions than cases where the slip direction is inferred as a line perpendicular to the s c intersection the filtering of data to reduce the scatter in the results produces insignificant improvement and is considered unnecessary

rb sr and k ar dating of clay diagenesis in jurassic sandstone oil reservoir north sea rb sr and k ar isotopic determinations were combined with scanning electron microscopy sem observations and x ray diffractometry xrd controls to date the diagenetic formation of clay minerals from oil bearing sandstones of the middle jurassic brent sandstone in the north sea the separated clay fractions often contain detrital components especially k feldspars which are reduced to sizes smaller than 0 4 m either by intense in situ alteration or by destructive sample preparation direct rb sr and k ar isotopic dating of diagenetic illite therefore was not possible isotopic data obtained on mixtures of detrital and newly formed components showed however that illite formed about 40 to 45 ma this value also was obtained directly on clay fractions separated by a gentle method of disaggregation illitization is related to emplacement of hydrocarbons and associated waters into the formation aqueous and hydrocarbon fluids simultaneously trapped in quartz overgrowth inclusions indicate late silicification contemporaneous with this fluid migration minimum trapping temperatures of at least 110 c for these fluid inclusions are too high for the present burial depth hot fluids are assumed to have originated in deep source rocks beneath the brent reservoirs and migrated upward during the middle eocene

re os geochronology and fingerprinting of united kingdom atlantic margin oil temporal implications for regional petroleum systems we demonstrate that the re os system can be used to understand the temporal evolution of a petroleum system as well as aid in the identification of source units traditional geochemical methods high performance liquid chromatography carbon isotope gas chromatography and biomarker analysis indicate that the main source of 18 oils analyzed for re os geochronology from the united kingdom atlantic margin clair schiehallion cuillin and foinaven fields is late jurassic marine shales this is supported by the os isotopic composition 187 os 188 os of the late jurassic source 0 9 2 4 at the timing of oil generation being similar to that of the oil 68 ma 0 92 1 12 indicating that the re os system can potentially be used to fingerprint the source of an oil the re os data for the 18 oils yield an age 68 13 ma that agrees with both the relative basin models and absolute ar ar geochronology timing of oil generation indicating that that re os oil geochronology records oil generation events

paleocave carbonate reservoirs origins burial depth modifications spatial complexity and reservoir implications paleocave systems form an important class of carbonate reservoirs that are products of near surface karst processes and later burial compaction and diagenesis features and origins of fractures breccias and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale pore networks and spatial complexities spatial complexities in paleocave reservoirs result from near surface and burial processes near surface processes include dissolutional excavation clastic sedimentation chemical precipitation and localized fracturing brecciation and collapse of cave walls and ceilings burial processes begin as cave systems subside into the subsurface remaining cave passages commonly collapse and early formed breccia clasts are rebrecciated differential compaction of strata around and over collapsed passages produces fractures crackle breccias and mosaic breccias near surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed paleocave systems the coalescing of passages in a cave system into larger connected porosity zones results from a combination of multiple cave forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured this combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across commonly forming large exploration targets final size pore network types and spatial complexities of coalesced collapsed paleocave systems are products of their evolution from near surface development through burial into the deeper subsurface the coalesced collapsed paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved

clumped isotope evidence for episodic rapid flow of fluids in a mineralized fault system in the peak district uk we have used clumped isotope thermometry to study a fault hosted hydrothermal calcite vein associated with the mississippi valley type mvt mineralization on the derbyshire platform in the southern pennines uk this is the first published dataset obtained using a new mass spectrometer mira optimized for clumped isotope analysis and an associated clumped isotope temperature calibration we analysed multiple generations of vein growth at high spatial resolution in two transects across the vein the vein grew episodically at temperatures between 40 and 100 c we interpret each episode of growth as being associated with an increasing flux of formation waters from deep sedimentary basins next to the mineralized platform and an accompanying increase in the precipitation temperatures heat is conserved in the fluid as it ascends along the fault surface and thus flow must have been fast and restricted to short lived pulses the flux could have been driven by high pore pressures associated with rapid sedimentation hydrocarbon generation and diagenesis in the basinal facies of the visean bowland hodder group natural hydraulic fracturing of shale units and failure of capillary seals possibly triggered by uplift allowed the release of fluids into aquifers within the sediment pile the transmission of high pore fluid pressures from the shales to the fault zone aided by the compressibility of the gas phase in two phase pore fluids may have resulted in fault rupture accompanied by enhanced fracture permeability and rapid fluid flow vein growth ceased as pore pressures dissipated such behaviour is closely related to a seismic valve type model for mineralization supplementary material details of the methods and equipment are available at https dx doi org 10 6084 m9 figshare 3808329 v9

genesis of cockade breccias in the tectonic evolution of the cirotan epithermal gold system west java the pliocene cirotan gold deposit provides an exceptional example of mineralized breccias and in particular cockade breccias analysis of these breccias approached through both field observation and image analysis on photographs made it possible to determine their method of graded formation the mineralized structure as much a 25 m thick in places is a right lateral strike slip fault that evolved to a normal fault at the end of the system s development it consists of juxtaposed breccia bodies separated by major slip or septa which formed at a major inflection of the host fault the breccia bodies all show a similar internal organization from footwall to hanging wall one passes from a siliceous breccia with angular clasts to a thick infill of cockade breccia showing a reverse grading with small centimetre size cockades aspect ratios of 1 10 passing to larger up to 1 m more equant cockades aspect ratios 1 3 in contact with the hanging wall the different stages of cockade formation demonst

a scale independent approach to fracture intensity and average spacing measurement fracture intensity the number of fractures per unit length along a sample line is an important attribute of fracture systems that can be problematic to establish in the subsurface lack of adequate constraints on fracture intensity may limit the economic exploitation of fractured reservoirs because intensity describes the abundance of fractures potentially available for fluid flow and the probability of encountering fractures in a borehole traditional methods of fracture intensity measurement are inadequate because they ignore the wide spectrum of fracture sizes found in many fracture systems and the consequent scale dependence of fracture intensity an alternative approach makes use of fracture size distributions which allow more meaningful comparisons between different locations and allow microfractures in subsurface samples to be used for fracture intensity measurement comparisons are more meaningful because sampling artifacts can be recognized and avoided and because common thresholds of fracture size can be enforced for counting in different locations additionally quantification of the fracture size distribution provides a mechanism for evaluation of uncertainties estimates of fracture intensity using this approach for two carbonate beds in the sierra madre oriental mexico illustrate how size cognizant measurements cast new light on widely accepted interpretation of geologic controls of fracture intensity

diagenetic k feldspar textures a tem study and model for diagenetic feldspar growth abstract diagenetic k feldspar overgrowths from the permo triassic continental clastic sediments of the elgin area in scotland were examined using conventional petrographic techniques and transmission electron microscopy to investigate the mechanisms of feldspar authigenesis the detrital k feldspar in these rocks is tartan twinned whereas the diagenetic k feldspar is untwinned the overgrowths are optically homogeneous but are not in optical continuity with the detrital grains overgrowths are up to 50 m thick and comprise numerous rhombs indicating that nucleation was a relatively easy process most overgrowths consist of a mosaic of micron sized rhombs nucleation on different twin planes on the detrital grain surface resulted in different orientations of the diagenetic nuclei the different orientations were propagated into the overgrowth as new nuclei formed on the surface of pre existing diagenetic crystals the detrital k feldspar is triclinic maximum microcline diagenetic k feldspar deviates only slightly from monoclinic orthoclase even though the triclinic polymorph is the stable k feldspar under the low temperature conditions of growth disordered crystals involve a simpler nucleation growth mechanism than ordered crystals consequently disordered crystals grow faster than ordered crystals monoclinic feldspar has disordered al si and triclinic feldspar has ordered al si diagenetic feldspar has metastable low triclinicity because this crystal structure could grow faster than the stable feldspar nanometer scale growth zones occur within individual rhomb representing variations in al si disorder such variations probably result from subtle variations in the state of supersaturation of the pore fluids and therefore in the rate of growth the interface between the detrital grain and the overgrowth represents no profound break in the feldspar crystal lattice despite the change in crystallography but is marked by the termination of twin planes in the detrital grain and the presence of dislocations in the overgrowth the termination of twins resulted in strain in the overgrowth which was relieved by the formation of dislocations small pores representing fluid inclusions and rare clay grains also occur at the interface

gallium germanium indium and other trace and minor elements in sphalerite as a function of deposit type a meta analysis abstract while a significant amount of analytical data on trace and minor element concentrations in sphalerite has been collected over the last six decades no meta analysis of this data has ever been conducted in this study the results of such an analysis are presented while the study focusses on ga ge and in data for six other elements ag cd co cu fe and mn was also included the results show that there are systematic statistically significant differences in the mean concentrations of fe ga ge in and mn in sphalerite from different deposit types while cd and cu concentrations show no such differences and ag and co concentrations are only significantly different for vein type deposits a principal component analysis demonstrates that the differences between deposit types are approximately one dimensional being expressible in terms of a single number this number correlates strongly with the homogenisation temperature of fluid inclusions r 2 0 82 p 16 it may be expressed as follows pc 1 ln c ga 0 22 c ge 0 22 c fe 0 37 c mn 0 20 c in 0 11 with ga ge in and mn concentrations in ppm and fe concentration in wt the relationship is sufficiently strong to be used as a geothermometer ggimfis the empirical relationship between pc 1 and the homogenisation temperature t is t c 54 4 7 3 pc 1 208 10 our results indicate a strong control of sphalerite chemistry by fluid temperature particularly for the concentrations of ga r 2 0 40 ge r 2 0 65 fe r 2 0 30 and mn r 2 0 60 and to a lesser degree in r 2 0 10 the concentrations of ag cd co and cu appear to be independent of temperature as a consequence of the strong temperature control on pc 1 metamorphic overprinting of pb zn deposits even by lower greenschist facies events may lead to significant changes in sphalerite composition namely a relative decrease in ga and ge concentrations and increase in fe mn and to a lesser degree in concentrations the closure temperature of sphalerite in regional metamorphic events appears to be around 310 50 c such that higher grade events will not be reflected in its composition factors other than temperature such as differences in fluid salinity or source rock composition do not appear to be responsible for differences between deposit types but rather appear to cause differences between individual deposits particularly the cu activity in ore forming systems appears to have a strong influence on in concentrations in sphalerite the observed trends in sphalerite compositions provide a useful tool for future studies of different types of pb zn deposits as well as for mineral exploration they should be particularly relevant for the identification of new resources of ga ge and in

wave field continuation and decomposition for passive seismic imaging under deep unconsolidated sediments abstract the coastal plains of the central and eastern united states contain deep sections of unconsolidated to poorly consolidated sediments these sediments mask deeper crustal and upper mantle converted phases in teleseismic receiver functions through large amplitude near surface reverberations thick sediments also amplify ambient noise levels to generally reduce data signal to noise ratios removing shallow sediment wave propagation effects is critical for imaging deep lithospheric structures a propagator matrix formalism is used to downward continue the wave field for teleseismic p waves into the midcrust and then to separate the upgoing s wave field from the total teleseismic response of the p wave to expose deep sp conversions this method requires that the earth model from the surface to the reference depth be known teleseismic p wave data for the memphis tennessee station mph are analyzed using a reference station deconvolution technique to produce vertical and radial p wave transfer functions these transfer functions are modeled using a simple model parameterization for sediment structure through grid inversion the inverted earth model is incorporated into the wave field continuation and decomposition technique to estimate the upgoing s wave field at 10 km depth in the crust moho and possible deeper ps conversions are identified with this process

u pb dating of calcite in ancient carbonates for age estimates of syn to post depositional processes a case study from the upper ediacaran strata of finnmark arctic norway results of in situ u pb dating of calcite spherulites cone in cone cic calcite and calcite fibres from a calcareous concretion of the upper ediacaran of finnmark arctic norway are reported ca

a review of molybdenite and fluorite mineralisation in caledonian granite basement western ireland incorporating new field and fluid inclusion studies and re os and u pb geochronology abstract the recent discovery of late magmatic quartz vein hosted molybdenite and exceptional gem quality vein fluorite in the caledonian galway granite complex ggc has prompted a review of these contrasting styles of mineralisation in the late caledonian granite basement connemara western ireland existing published u pb and re os chronometry and fluid inclusion microthermometry are combined with new a geological field observations b u pb zircon and re os molybdenite geochronometry and c fluid inclusion microthermometry to generate a new pressure temperature time model p t t of mineralisation for the ggc re os chronometry molybdenite indicates that granite related molybdenite mineralisation extended from 423 to 380ma overlapping with the ggc emplacement history determined by u pb zircon chronometry the p t t model reflects initial granite emplacement and mo mineralisation at 423ma followed by lower p and t granite emplacement and related quartz vein hosted mo mineralisation at 410ma carna pluton 400ma kilkieran pluton and at 380ma costelloe murvey granite the gem quality fluorite veins in the ggc represent late triassic hydrothermal mineralisation these veins form part of a regional n atlantic european triassic jurassic hydrothermal mineralisation province vein emplacement was triggered by the rifting of the n atlantic facilitating crustal thinning and subsidence of continental crust and initiating hydrothermal activity at the margins of mesozoic basins

local earthquake wave propagation through mississippi embayment sediments part i body wave phases and local site responses p and s body waves from microearthquakes in the new madrid seismic zone nmsz are investigated at selected sites in an effort to understand wave propagation from future large earthquakes earthquake body waveforms display distinctive features that constrain the nature of p and s wave local site responses and wave propagation within the unconsolidated mississippi embayment sediments modeling of the waveforms demonstrates that a near surface low velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large p and s wave resonance effects that can be used to infer near site conditions site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies site resonance affects p and s wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters travel times of observed body wave phases such as p ppphp the first p wave reverberation within the entire sedimentary column ps sp s and ssphp can be used to estimate the average wave slownesses and poisson s ratio within the embayment sediments an average poisson s ratio of 0 44 is obtained for the central nmsz under station pebm detailed s wave velocities are derived for a nafe drake sediment model using acoustic well logs and the travel time constraints of observed seismic phases v p v s ratios vary from 5 5 near the surface to approximately 2 4 at the base of the sediments use of the well log data in wave calculations also explains much of the nature of p and s wave coda within the waveforms and shows that 1d heterogeneity is a first order influence on seismic wave propagation within the mississippi embayment manuscript received 11 march 2003

natural fracture propping and earthquake induced oil migration in fractured basement reservoirs the geological processes that create fluid storage capacity and connectivity in global fractured basement reservoirs are poorly understood compared to conventional hydrocarbon plays hosting potentially multibillion barrels of oil the upfaulted precambrian basement of the rona ridge offshore west of shetland uk gives key insights into how such reservoirs form oil presence is everywhere associated with sub millimeter to meterthickness mineralized fracture systems cutting both basement and local preseal cover sequences mineral textures and fluid inclusion geothermometry suggest a low temperature 90 220 c near surface hydrothermal system as does the preservation of clastic sediments in the same fractures these fills act as permanent props holding fractures open forming long term fissures in the basement that permit oil ingress and storage calcite fill u pb dating constrains the onset of mineralization and contemporaneous oil charge to the late cretaceous the additional preservation of oil stained injected sediment slurries and silica gels along basement faults suggests that rift related seismogenic faulting initiated lateral oil migration from jurassic source rocks into the adjacent upfaulted ridge subsidence below sea level in the latest cretaceous sealed the ridge with shales and buoyancy driven migration of oil into the preexisting propped fracture systems continued long after the cessation of rifting these new observations provide an explanation for the viability of sub unconformity fractured basement reservoirs worldwide and have wider implications for subsurface fluid migration processes generally

microtremor observations of deep sediment resonance in metropolitan memphis tennessee we measured site resonance periods throughout metropolitan memphis tennessee the site resonance was derived from recordings of ambient microtremor and is a direct measurement of seismic ground shaking in the memphis metropolitan area the period t0 in s is predicted by the empirical relation t0 0 39899 7 8922 10 3h 3 0634 10 6h2 350 h 1100 m where h is the thickness in m of the unconsolidated sediments that overly hard paleozoic rocks assuming that a quarter wavelength relationship governs the relationship between resonant period sediment thickness and average shear wave speed we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses vs 521 15 0 37459h 350 h 1100 m the scatter in the observations and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

k ar dating of time of gas emplacement in rotliegendes sandstone netherlands the time of emplacement of gas into each of two reservoirs in the rotliegendes sandstone one in the groningen field and one in the broad fourteens basin was estimated from potassium argon k ar ages of diagenetic illitic clays and careful petrographic observations of the rocks in which the clays occurred in each reservoir the intensity of diagenetic alteration increased downward within a relatively thin 15 to 175 m or 50 to 580 ft interval apparently reflecting a longer period of alteration for deeper samples this is expected in cases where a gradually accumulating gas column expels pore water from progressively deeper parts of the reservoir rock illite is a common diagenetic phase in the rotliegendes because detrital contamination in these rocks has been infer ed to be minimal k ar ages of very fine grained illite should indicate the time the illite forming process ceases in each case reported here the age of the deeper sample is younger than that of the shallower we interpret the age of the finest illite fraction in the gas zone of each reservoir to indicate the time at which illite formation ceased as the result of gas emplacement and pore fluid displacement in the rock our results indicate that gas emplacement in the groningen field commenced prior to 150 ma and that the growth of diagenetic illite continued in the present day water zone from that time until at least 120 ma

evaluation of deep sediment velocity structure in the new madrid seismic zone detailed knowledge of the physical properties of the sediments filling the mississippi embayment has proven critical to both unravel the tectonic frame work operating in the region and assess the seismic hazards posed by the new madrid seismic zone in this article we show that independent geotechnical estimates for p and s wave velocities are compatible with a sedimentary model of k feldspar clasts embeded in water and we test its validity by modeling receiver functions at a number of broadband stations by constraining the bulk sediment thicknesses beneath each station from independent reflection profiling estimates we have been able to recover the depth to the top of the cretaceous from the receiver function data at individual stations our receiver function modeling thus provides confidence in the velocity and density structures extrapolated from in situ geotechnical measurements in the upper mississippi embayment

fractured basement play development on the uk and norwegian rifted margins fractured crystalline basement reservoirs basement on the uk continental shelf ukcs and the norwegian continental shelf ncs have been underexplored over the last 12 years hurricane energy has deliberately set out to explore basement potential by exploring and appraising the rona ridge west of shetland and the rona ridge lancaster field is now being progressed towards being the first uk basement field development the norwegian basement play is also recognized as a potentially material resource through serendipitous oil discoveries with the 16 1 15 well drilled in 2011 being the first successful full scale basement test on the ncs building on this success the 2018 rolvsnes appraisal well 16 1 28 s has demonstrated the significant basement potential of the extensive utsira high and confirmed the materiality of a norwegian basement play the rona ridge and utsira basement discoveries are used as a comparison with two other yet to be evaluated prospective basement plays on the ncs with the objective of establishing technical subject areas where future ukcs and ncs collaboration would aid in accelerating understanding of the ukcs ncs basement play

quantifying fault breccia geometry dent fault nw england a geometric classification of fault breccia borrowed from the cave collapse literature has been suggested as an alternative to available genetic classifications here image analysis is used to explore geometric discrimination between the visually assigned classes of crackle breccia mosaic breccia and chaotic breccia using samples from the well understood dent fault northwest england clast sphericity and surface roughness show some correlation with the breccia classes but particle size distributions and their fractal dimension show none a more useful parameter is the percentage of sample area occupied by clasts crackle breccia has 75 clasts mosaic breccia 60 75 clasts and chaotic breccia has 60 clasts the eye is also good at judging the tessellation goodness of fit of clasts and a semi quantitative approach to assessing this parameter is explored the average degree of rotational misfit of the clasts is strongly related to breccia class crackle breccia involves less than 10 average rotation mosaic breccia 10 20 and chaotic breccia more than 20 rotation comparison charts are provided for semi quantitative classification of fault breccias

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

sedimentary hosted polymetallic massive sulfide deposits of the kebrit and shaban deeps red sea massive sulfides recovered from the kebrit deep carbonaceous sedimentary succession represent black smoker fragments novel to any red sea brine pool deposit chimneys which were also observed in situ near the seawater brine interface of the kebrit deep pool are primarily comprised of fe zn and pb bearing phases and are often tar and asphalt impregnated cu sulfides are virtually absent from parageneses contrasting rift related smoker and red sea metalliferous sediment deposits concentration of nickel in discrete bravoite points to a basalt seawater leaching process as a source for most metals the sedimentary package which probably hosts cu mineralization in lower stockworks of the smoker deposit is considered the major source of lead prevention of boiling of hydrothermal fluids passing through a succession of organic rich carbonate and clay horizons prior to discharge is essential for smoker formation shaban deep sedimentary hosted massive sulfides are less frequent with pyrite being the dominant ore mineral sulfur isotope data indicate both high temperature inorganic as well as biogenic sulfate seawater and or evaporite reduction in sulfide forming processes cogenetic sulfates formed from residual bacteriogenically reduced seawater sulfate rather low sulfide sulfate precipitation temperatures of 110 130 c for the kebrit brine pool and 100 c for shaban deep massive sulfides are evident

some aspects of the recent tectonics in the strait of messina italy abstract during pliocene and pleistocene times the strait of messina was subjected to extensional tectonism which was contemporaneous with marine sedimentary processes various tectonic sedimentary features created by this combination of processes are reviewed depocentres synsedimentary truncations and the association of sedimentary dykes related to extensional fissuring and hydraulic fracturing these features are regarded as characteristic accompaniments to extensional block faulting the recent structural evolution of the strait of messina margins provides an illustration of high magnitude vertical movements occurring within a short time interval the subsidence phase is related to fault movements which generate important fault scarps and sedimentary dykes phases of upheaval in contrast correspond to a general uplift of the margins unrelated to the local fault movements

the nature and age of basement host rocks and fissure fills in the lancaster field fractured reservoir west of shetland hosting up to 3 3 billion barrels of oil in place the upfaulted precambrian crystalline rocks of the lancaster field offshore west of shetland give key insights into how fractured hydrocarbon reservoirs can form in such old rocks the neoarchean c 2700 2740 ma charnockitic basement is cut by deeply penetrating oil mineral and sediment filled fissure systems seen in geophysical and production logs and thin sections of core mineral textures and fluid inclusion geothermometry suggest that a low temperature supplementary material analytical methods and isotopic compositions and ages are available at https doi org 10 6084 m9 figshare c 4763237 thematic collection this article is part of the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

hydrothermal fluid origins in a fluorite rich mississippi valley type district combined noble gas he ar kr and halogen cl br i analysis of fluid inclusions from the south pennine ore field united kingdom the south pennine peak district ore field united kingdom is host to several fluorite rich mississippi valley type deposits and was chosen for study as an analogue to the illinois kentucky fluorspar district united states the study was undertaken to test the idea that the large scale carbonate dissolution and fluorite rich mineralogy characteristic of both districts may require a magmatic source of hf the data also provide constraints for water rock interactions and potential fluid sources relevant to the local geology of the south pennine ore field samples were studied from two of the largest and most economically important deposits within the south pennine ore field the hucklow edge and dirtlow rakes in vacuo crushing of irradiated calcite samples released inclusion fluids that were analyzed for noble gas isotopes ar ar kr and the halogens cl br i this was achieved simultaneously by noble gas mass spectrometry ar ar methodology and was combined with fluid inclusion thermometric salinity data to determine noble gas concentrations in addition he isotope analyses were obtained from unirradiated fluorite the he he ratio of the pennine ore fluids 0 1 ra where ra atmospheric he he ratio of 1 4 10 is significantly lower than the maximum value recorded in the illinois kentucky fluorspar district 0 35 ra kendrick et al 2002 this volume indicating a crustal fluid source and supporting existing basinal brine models the low ar ar ratios with most 50 to 60 percent halite dissolution water br cl mol ratio 0 57 0 92 10 the salinity of fluids included in the main stage of the hucklow edge mineralization avg of 22 wt nacl equiv places an upper limit on the extent of meteoric recharge at 15 vol percent a secondary vein present at hucklow edge has a lower salinity of 40 m y a time scale compatible with models in which fluids were derived from small local basins such as the widmerpool and edale gulfs

the nature and significance of the faroe shetland terrane linking archaean basement blocks across the north atlantic abstract core samples of the continental basement rocks that underlie the eastern faroe shetland basin fsb and its inshore margins west of shetland reveal a suite of predominantly granodioritic to granitic orthogneisses including ttg together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites a small area of lithologically similar gneisses also crops out onshore at north roe uyea west of the caledonian front in shetland in the core samples coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak but ubiquitous static greenschist facies retrogression later structures include widespread epidote quartz veining and local developments of mylonite cataclasite pseudotachylite and phyllonite regions associated with the rona ridge oil fields e g clair lancaster also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization quartz adularia calcite pyrite chalcopyrite with significant fracture hosted hydrocarbons new and published u pb zircon analyses from the gneisses offshore give a relatively narrow range of neoarchaean protolith ages ca 2 7 2 8 ga spread over a geographic area of 60 000 km2 west of shetland detrital zircon data from overlying triassic cretaceous sedimentary sequences in the fsb suggest an equivalent limited range of neoarchaean source materials hf isotopic data indicate involvement of palaeo to mesoarchaean crustal sources overall our findings suggest that a major phase of neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region they are similar in age and lithology to the protoliths of the nearest onshore lewisian complex of nw scotland rhiconich terrane however they lack geochronological or textural evidence for the widespread proterozoic laxfordian events ca 1 7 1 8 ga which are widespread in scotland this suggests that the precambrian rocks west of shetland the faroe shetland terrane can be correlated with the archaean rocks of the central greenland rae craton and that a northern limit of proterozoic reworking lies just offshore from the north coast of scotland

crystal growth diagrams as an aid to interpreting the fabrics of calcite aggregates abstract the morphological and optical properties of crystal aggregates which develop by nucleated growth from a plane substrate are modelled by crystal growth diagrams such diagrams show that fabric maturation occurs through three growth stages 1 isolated 2 competitive and 3 parallel diagrams are presented which are constructed using calcite rhombohedra of equant 1011 obtuse 0112 and acute 4041 form the acute form develops in the most mature parallel growth stage length fast columnar crystals this graphical model is matched by a natural calcite cemented neptunian dyke the obtuse form develops in the most mature parallel growth stage approximately length slow bladed crystals this graphical model is matched by a natural calcite cemented dilational vein diagrams are presented which show seeded in contrast to nucleated growth seeding produces overgrowths which are divisible into an early epitaxial stage of rapid growth and a later idiomorphic stage the volume of epitaxial growth depends on surface area shape and lattice perfection of the seed and on the crystal habit of the overgrowth the overriding importance of echinoderm skeletal elements in stimulating epitaxial growth in limestones is related to their large size relative to other seeds and to their lattice properties polycrystalline seeds with parallel c axis orientation develop overgrowth aggregates which omit the competitive stage because the crystals are constrained to develop normally to seed surfaces even when these are not the directions of most rapid growth growth from polycrystalline seeds with random c axis orientation produce crystal aggregates similar to those developed by nucleation from a passive substrate crystal growth diagrams are presented to show the effect of a change in crystal habit during continuous growth the acute to obtuse change examined results in the crystal s greatest growth vector s being reorientated which in turn causes a new episode of fabric maturation this graphical model is matched by a calcite cement from a south wales grainstone t shaped crystals growth hiatuses on some crystal surfaces and enfacial junctions are created in both model and natural example the use of crystal fabrics in the diagnosis of cement the correlation between crystal fabric and saturation state and the standard of crystallographic observation in studies of calcite cement are criticized the match between accurately drawn crystal growth diagrams and natural crystal aggregates is excellent these diagrams are an underused aid in the stereological assessment of crystal aggregates and reveal features that are not intuitively obvious

evidence for late paleozoic brine migration in cambrian carbonate rocks of the central and southern appalachians implications for mississippi valley type sulfide mineralization abstract many lower paleozoic limestones and dolostones in the valley and ridge province of the central and southern appalachians contain 10 to 25 weight percent authigenic potassium feldspar this was considered to be a product of early diagenesis however 40 ar 39 ar analyses of overgrowths on detrital k feldspar in cambrian carbonate rocks from pennsylvania maryland virginia and tennessee yield late carboniferous early permian ages 278 322 ma simple mass balance calculations suggest that the feldspar could not have formed isochemically but required the flux of multiple pore volumes of fluid through the rocks reflecting regional fluid migration events during the late paleozoic alleghanian orogeny microthermometric measurements of fluid inclusions in overgrowths on detrital k feldspar and quartz grains from unmineralized rocks throughout the study area indicate homogenization temperatures from 100 to 200 c and freezing point depressions of 14 to 18 5 c 18 21 wt nacl equiv the apparent similarity of these fluids to fluid inclusions in ore and gangue minerals of nearby mississippi valley type mvt deposits suggests that the regional occurrences of authigenic k feldspar and mvt mineralization may be genetically related this hypothesis is supported by the discovery of authigenic k feldspar intergrown with sphalerite in several mines of the mascot jefferson city district e tennessee regional potassic alteration in unmineralized carbonate rocks and localized occurrences of mvt mineralization are both explainable by a gravity driven flow model in which deep brines migrate towards the basin margin under a hydraulic gradient established during the alleghanian orogeny the authigenic k feldspar may reflect the loss of k during disequilibrium cooling of the ascending brines mvt deposits are probably localized manifestations of the same migrating fluids occurring where the necessary physical and chemical traps are present

chaotic breccia along the dent fault nw england implosion or collapse of a fault void a body of chaotic breccia along the reverse oblique dent fault zone is ascribed to hanging wall collapse into persistent voids created by geometric mismatch of fault walls although some implosion into transient voids is a possibility the breccia comprises a 20 m wide body of hanging wall lithologies with a chaotic clast supported fabric that contrasts with the fitted fabric breccias typical of the dent fault damage zone the breccia body has crude bedding defined by clast shape and size contrasts the void fill is cut by variscan fault strands which together with its ferroan calcite and barite cement prove its late carboniferous rather than recent age it is shown that any fault void transient or persistent had a smaller aperture than the final width of the breccia body and no more than 5 m a span that can be supported to depths of 2 or 3 km however cement zonation in the breccia fill suggests that the void opened in multiple increments each of an aperture compatible with the maximum displacement in any one event along the dent fault the dent fault example highlights the possible general importance of fault void collapse but also the problems in distinguishing it from implosion processes

modeling petroleum expulsion in sedimentary basins the importance of igneous intrusion timing and basement composition we thank siccar point energy ltd for permission to publish this study thanks also to robert holdsworth simon holford and christian huag eide for their insightful feedback which has greatly improved the manuscript 1gsa data repository item 2019328 supplementary data geochemical summary rationale and data tables is available online at http www geosociety org datarepository 2019 or on request from editing geosociety org

paleomagnetic dating of fracturing using breccia veins in durness group carbonates nw scotland abstract a paleomagnetic study of red fault related breccia veins in the cambro ordovician durness group in nw scotland was conducted to determine the time of brecciation the origin of the veins and the nature and timing of associated fluid related diagenetic alteration the veins contain brecciated fragments of the host durness group and strike either east west or north south clasts of breccia cemented by calcite suggest multiple brecciation events the host durness limestone is a gray dolomite and contains a devonian chemical remanent magnetization crm that resides in magnetite the veins contain magnetizations that reside in hematite and are interpreted as crms the breccias in north south veins contain a triassic crm whereas the veins with east west strikes contain a jurassic crm authigenic hematite is common in the breccias along growth planes in the calcite cements the two crms within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite the brecciation and fluid flow events are interpreted to be related to extension in the mesozoic which is consistent with the extensional history of the northern atlantic margins

cockade breccia product of mineralisation along dilational faults cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement they occur particularly in low temperature near surface hydrothermal veins at least six mechanisms have been proposed for the formation of cockade breccia like textures but only two repeated rotation accretion and partial metasomatic replacement of clast minerals have been supported by detailed evidence a typical example of cockade breccia from the gower peninsula south wales shows clear evidence for the rotation accretion mechanism in particular overgrown breakage points in cement layers where cockades were previously touching each other and rotated geopetal infills of haematitic sediment based on the available evidence it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip seven criteria are given for the correct identification of cockade breccias

chaotic breccia zones on the pembroke peninsula south wales evidence for collapse into voids along dilational faults chaotic breccias and megabreccias locally called gash breccias hosted within the pembroke limestone group visean mississippian lower carboniferous of southwest wales are re mapped along with spatially related crackle and mosaic breccias of thirteen studied megabreccia bodies seven lie along steep nnw or nne striking strike slip faults originating during north south variscan late carboniferous shortening though reactivated during later extension four bodies are conformable with e w striking steeply dipping bedding and two have irregular or indeterminate margins the bedding parallel zones are interpreted as the dilational tips of listric normal faults and the cross strike faults as transtensional transfer zones sub horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults most fissures have geometrically matched margins produced by this dilational faulting and only locally have the indented margins indicating solutional processes the most likely age for the main fissure extension and fill is late triassic based on analogous dated fills at the eastern end of the bristol channel basin the pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones

structural controls and origin of gold silver mineralization in the grampian terrane of scotland and ireland gold bearing mineral deposits occur over a strike distance of 300 km within the grampian terrane of scotland and ireland this terrane consists of neoproterozoic lower ordovician rocks of the dalradian supergroup that were polyphase deformed and metamorphosed during the c 470 ma grampian orogeny sulphide rich au ag deposits occur in scotland at calliachar urlar burn tombuie tyndrum and cononish and in ireland at curraghinalt omagh cavanacaw croagh patrick cregganbaun and bohaun they are hosted by 0 1 6 m thick quartz veins and have a similar overall mineralogy including native gold as cu fe pb and sn sulphides with hessite tetrahedrite and electrum present in the first six localities above the mineralized quartz veins which are characterized by open space textures crystallized at c 3 5 km depth in the crust all of the deposits were structurally controlled and apart from curraghinalt occur within second order riedel r r and t fractures resulting from a regional n s trending maximum principal stress these deposits are of upper silurian to lower devonian post scandian age and are inferred to have crystallized from hot silica rich metamorphic fluids derived from dehydration reactions at the greenschist amphibolite facies boundary curraghinalt is an older grampian thrust related deposit plutonic igneous rocks mainly granitoid contributed in part to the fluids which were channelled into major orogen parallel strike slip faults to be injected by fault valve pumping into the damage zones and fault breccias of newly formed riedel fractures any residual fluid probably percolated to the ground surface to form rhynie chert type hot springs

dating of multistage fluid flow in sandstones through ultraviolet laser argon argon dating of potassium feldspar cements containing fluid inclusions we determined temperature composition time data for paleofluids in a sedimentary basin including data for an evolving episode of fluid flow recorded in distinct phases of cement the fluid evolved from mixed aqueous oil 83 million years ago to purely aqueous by 76 million years ago thus dating the time of oil charge in this reservoir

the development of cavities and clastic infills along fault related fractures in tertiary basalts on the ne atlantic margin field based geological observations have revealed the hitherto unrecognised development of post magmatic brittle deformation structures cutting tertiary volcanic rocks in the faroe islands these faults and fractures are characteristically associated with different styles of clastic sedimentary infill including 1 0 3 1 0 m thick clastic units infilling open fractures formed along pre existing steeply dipping to sub vertical faults 2 0 1 0 6 m thick sub horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts 3 anastomosing mm scale and planar dm scale clastic intrusion features mobilised and emplaced during transient fault related overpressuring events along pre existing fractures cutting the surrounding volcanic units the infill features provide evidence for the existence of sustained open cavities in the sub surface the clastic materials are commonly internally affected by later fault related deformation and lack mineralisation unlike all preceding faulting episodes in the faroes region perhaps reflecting their near surface development we believe structures equivalent to these features may occur widely in other parts of the ne atlantic margin particularly along the outer arcs of gentle regional scale fold hinges the uncemented fracture hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability cenozoic volcanic sequences of the ne atlantic region

the nature and significance of rift related near surface fissure fill networks in fractured carbonates below regional unconformities fissure fill networks are a widely recognized but relatively little described near surface phenomenon supplementary material appendix a is available at https doi org 10 6084 m9 figshare c 5023103

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

velocity structure of the northern mississippi embayment sediments part ii inversion of teleseismic p wave transfer functions abstract detailed shear and compressional gradient velocity structure for the unconsolidated sediments of the mississippi embayment me have been obtained through simultaneous inversion of radial and vertical teleseismic transfer functions for 60 broadband stations inside the me transfer functions were calculated by deconvolving a vertical array beam from the radial and vertical displacement at each station inside the embayment the vertical array beam was constructed by stacking all the vertical components of motions for stations on the bedrock and outside the embayment to reduce the effect of incoherent scattering improving estimates of the source time function and stabilizing the deconvolution a bounded variable least squares inversion technique was applied to invert for velocity nodes on the top and bottom of the sediment layer p wave velocity changes from 1 0 km s near the surface and increases with depth from 3 5 to 4 km s in deeper parts s wave velocity varies from 0 3 km s and increases to 1 6 km s in deeper sections to the south electronic supplement gradient velocity structure of the northern mississippi embayment sediments for 60 broadband stations

the age origin and tectonic significance of mesozoic sediment filled fissures in the mendip hills sw england implications for extension models and jurassic sea level curves in the eastern mendip hills on the northern margin of the wessex basin sw england the carboniferous limestone is cut by numerous fissures that are filled with mesozoic sediments sedimentary dykes neptunian dykes the fissures contain a record of triassic lower jurassic sediments that are only sparingly preserved in their normal stratigraphical position between the carboniferous limestone and the unconformably overlying upper inferior oolite of bajocian age detailed analysis of cross cutting relationships facies analysis biostratigraphy lithostratigraphy and strontium isotope ages of relevant mesozoic sediments has allowed the construction of an upper triassic lower jurassic fissure fill stratigraphy for the eastern mendip area most fissures were clearly formed by rapid influx of unlithified sediment from the land surface or sea floor some smaller cavities or larger cavities with restricted access to the unconformity were apparently filled by sediment that trickled down into the fissure system the vast majority of the mendip fissures are interpreted as having formed as a response of the carboniferous limestone north of major basin bounding faults to pulses of tectonic extension during ladinian norian rhaetian late hettangian early sinemurian late sinemurian early pliensbachian late pliensbachian and bajocian times triassic earliest jurassic fissures have a broad spread of strike from e w to nw se to n s accommodating extension in a roughly ne sw direction younger jurassic fissures show well defined e w and n s trends with the former becoming dominant through time total extension of 4 7 n s and 0 6 e w was produced by the formation of triassic jurassic fissures within the carboniferous limestone such patterns of extension are thought likely to be characteristic of the subsurface geology in much of southern england and wales major implications of this study are that 1 the presence of seismically unresolvable sediment filled fissures in supposedly rigid fault blocks can lead to a significant underestimate of regional extension based on the restoration of motion on normal faults on seismic reflection profiles and 2 the isolation of pulses of tectonic activity with a temporal resolution of 10 5 10 6 years may provide a means of identifying a tectonic signal in relative sea level curves derived from the jurassic sedimentary record

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

shear wave velocity of the sedimentary basin in the upper mississippi embayment using s to p converted waves from mid october 1989 to august 1992 40 three component panda portable array for numerical data acquisition stations were deployed in the central new madrid seismic zone three component digital seismograms recorded by the panda stations in the region are characterized by 1 the very weak direct s arrivals on the vertical component which can be identified unambiguously from the two horizontal components and 2 at least two prominent secondary arrivals between the direct p and s arrivals one sp dominant on the vertical component and another ps with smaller amplitude on the two horizontal components travel time differences between the sp and s and between the p and ps are the same for different earthquakes recorded at the same station but are different at different stations even for the same event polarization analyses of three component seismograms and travel time measurements confirm the interpretation that these two secondary arrivals are the p to s ps and s to p sp converted waves that occur at the bottom of the sedimentary basin beneath each station since abundant well log data are available in the upper mississippi embayment the thickness of the sediments beneath each seismic station can be estimated travel time differences between the direct and the converted waves can be used to calculate average shear wave velocity for the sediments beneath each station the estimated shear wave velocities of the sediments beneath panda stations vary from 0 45 to 0 67 km sec the higher shear wave velocity associated with thicker sediments can be interpreted as a consequence of increasing compaction of unconsolidated sediments due to increasing overburden

array observations of microseismic noise and the nature of h v in the mississippi embayment abstract ambient ground motion data were collected using phased seismic arrays in fall 2002 and spring 2007 within the mississippi embayment and at a single station external to the embayment these data allowed us to determine the wave field composition of ambient noise for understanding wave propagation mechanisms giving rise to spectral peaks using nakamura s h v technique ambient ground motions in the frequency band of 0 1 0 33 hz 10 3 sec period were dominated by spatially localized rayleigh and love wave microseisms generated by high ocean waves along the north american seaboard in the time periods of analysis seismic waves important in forming the h v peak near 4 sec period are composed of relatively high phase velocity rayleigh and love waves that convert to propagating homogeneous shear waves in the thick unconsolidated sediments of the embayment the h v resonant period is controlled by both constructive and destructive interference of these shear waves a simple relationship for the h v peak is given using a propagator matrix formulation that predicts the resonance frequency of a layered medium for surface wave motion at the base of the system the amplitude of the observed h v peak however does not give an accurate estimate of shear wave amplification because it depends on the slowness of the incident wave the inconsistency in estimated average shear wave velocities using the h v method and differential travel times of local earthquake sp phases in the mississippi embayment may be explained by misidentification of sp wave conversion points from deeper interfaces

basement reservoir plumbing fracture aperture length and topology analysis of the lewisian complex nw scotland upfaulted ridges of neoarchean crystalline basement rocks formed in the faeroe shetland basin as a consequence of mesozoic rift processes and are an active target for oil exploration we carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in nw scotland lewisian gneiss complex as an analogue for the offshore oil and gas reservoirs of the uplifted rona ridge basement high our analysis shows a power law distribution for fracture sizes aperture and length with random to clustered spacing and high connectivity indices regional variations between the scottish mainland and the outer hebrides are recognized that compare directly with variations observed along the rona ridge in the faeroe shetland basin here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity more generally the combination of size spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective well plumbed reservoir systems in their own right supplementary material additional methods and results are available at https doi org 10 6084 m9 figshare c 5017139 thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

relationship between karstification and burial dolomitization in permian platform carbonates lower khuff oman large breccia fabrics associated with karst constitute an important structure in massive limestone successions the dimensions and shapes of breccia structures are controlled by the initial fracture pattern of the limestone and preferential pathways of the karstifying fluids but subsequently breccia fabrics can also govern the migration of later fluids therefore breccias are highly relevant features to capture for reservoir characterisation outcrop analogues for lower khuff units in the middle east present in the central oman mountains reveal brecciated fabrics up to 10 s of meters in diameter these brecciated units are closely associated with dolomite bodies of late diagenetic origin based on an integrated set of data the breccias are interpreted as collapsed karst cavities either formed by meteoric or hypogenic fluids the exact origin of the fluids could not be constrained due to an overprint by later dolomitizing fluids based on the composition of the clasts and matrix in the breccias two dolomitization events are interpreted to have affected the succession one prior to early diagenetic ed dolomite and one after brecciation late diagenetic dt2 dolomite dolomite of shallow burial origin ed dolomite was only observed as clasts within breccia and is much more frequent than late diagenetic medium to deep burial dolomite clasts thus the timing of the brecciation and collapse is assumed to postdate shallow burial early diagenetic dolomitization late diagenetic replacive dolomite dt2 dolomite forms 90 of the matrix in the breccia fabrics with the exception of a small area that was not affected by dolomitization but is rarely present as clasts stable isotope measurements 18o 2 5 to 6 vpdb and 13c 2 9 to 4 8 vpdb suggest a burial origin for the late diagenetic dolomite potentially with the participation of hydrothermal fluids the dolomitized matrix indicates a migration of late dolomitizing fluids subsequent to or postdating the collapse of the karstic cavities thus early karstification processes seem to have played a big role in controlling subsequent loci of late dolomitization in the oman mountains and potentially in other similar settings elsewhere

mn and fe rich black travertine shrubs bacterially and nanobacterially induced precipitates abstract black shrubs i e arborescent forms are present within hot water travertine deposits from morocco the shrubs are commonly a few centimeters high and laterally grouped along individual laminae other than in mineral composition the black shrubs closely resemble the calcite shrubs from other hot water travertine deposits e g bagni di tivoli italy and mammoth hot springs yellowstone national park they are composed of a variety of minerals that are mn rich and also have a significant fe oxide component similar mn and fe rich shrub like forms have been described from desert varnish deep sea mn nodules and marine carbonates frutexites sem analyses show that bacterial bodies are densely packed within the mn rich black travertine shrubs whereas no bacterial bodies are evident within the enclosing aragonite and calcite laminae the bacterial bodies range in shape and size from rods commonly 0 8 m long by 0 1 m in diameter to spherical forms commonly 0 3 m in diameter that form chains up to 1 m long to nanobacterial spheres 150 to 200 nm in diameter that make up densely packed chains commonly 15 m in length it is well documented within the microbiological literature that not only do bacteria have the ability to induce the precipitation of mn and fe they can concentrate mn in mineral precipitates from waters with very low to essentially undetectable concentrations of mn the black mn rich shrubs within the moroccan travertines other hot water travertines and related features such as desert varnish deep sea mn nodules and frutexites are postulated to also be the result of bacterially induced precipitation

three dimensional seismic velocity model for the unconsolidated mississippi embayment sediments from h v ambient noise measurements abstract the horizontal to vertical h v technique by nakamura 1989 was applied to data from 30 new field stations and 64 other broadband temporary and permanent seismic stations within the mississippi embayment of the central united states to develop a 3d model of unconsolidated sediment shear wave velocity structure using the dart 1992 map of sediment thickness as a basis two self consistent models of average shear wave velocity versus sediment thickness were developed by utilizing the theoretical linear relationship between the frequency of the h v peak and shear wave velocity one model was based on the observation that the h v peak period t p s versus sediment thickness h m was seen to be approximately linear with the relationship t p 0 003266 h 1 084 the second model was developed by considering peak frequency f p versus sediment thickness parameterized to follow ln f p 8 325 10 7 h 2 0 00232 h 0 01796 overall the models show low average shear wave velocity near the edge of the mississippi embayment with velocities increasing with increasing sediment thickness consistent with increased sediment compaction these models will be useful in studies of site resonance and amplification for earthquake shaking hazards and for wave propagation computations for the region

long lived crustal scale fluid flow the hydrothermal mega breccia of hidden valley mt painter inlier south australia the palaeozoic hidden valley breccia in the northern flinders ranges of south australia is exceptional for 1 its size of about 10 km2 2 the large continuous range in clast sizes from tens of microns to hundreds of metres and 3 thorough mixing of lithologies of different provenance some originally kilometres apart stratigraphically the size distribution follows a single fractal dimension of about unity over at least 6 orders of magnitude implying that a single process was responsible for diminution from the 100 m scale down to 12 km exhumation which lasted about 200 myrs starting during the 500 ma delamerian orogeny and continuing during the alice springs orogeny fluids released during exhumation were structurally focussed towards hidden valley where an estimated 20 5 30 km3 total fluid volume caused the extensive brecciation brecciation initiated in neoproterozoic cover metasedimentary rocks at a level that is now fully exhumed as hydrothermal fluid ascent continued with ongoing exhumation the level of brecciation shifted down into the underlying mesoproterozoic basement rocks taking with it clasts of cover rocks in this model rocks pass through the zone of brecciation which can thus incorporate a variety of lithologies that were originally kilometres apart

velocity structure of the northern mississippi embayment sediments part i teleseismic p wave spectral ratios analysis abstract the mississippi embayment overlies the new madrid seismic zone in the central united states and consists of a thick succession of unconsolidated sediments that covers the paleozoic basement rock the high velocity contrast at the basement sediment interface and the near vertical ray paths of teleseismic p waves generate very large p to s conversions on the radial component and large p reverberations on the vertical component this characteristic of p wave propagation is used to study p and s wave resonance in the sedimentary layer horizontal to vertical h v and vertical to horizontal v h power spectral ratios are calculated for a time window around the teleseismic p wave arrivals at broadband stations of the usarray transportable array northern embayment lithosphere experiment and the new madrid cooperative seismic network in the embayment using a map of sediment thickness we developed models for average p and s wave velocity versus sediment thickness the resulting shear wave velocity model matches very well with the model obtained from an h v ambient noise study in the region the shear wave velocity model shows low velocity near the edges of the embayment with average velocities increasing with increasing sediment thickness consistent with increased sediment compaction fundamental resonance frequency for s waves ranges from 0 2 to 0 4 hz for the embayment sediments electronic supplement two figures showing the synthetic test results and a table that includes peak resonance frequency and average v p and v s

breccia formation by particle fluidization in fault zones implications for transitory rupture controlled fluid flow regimes in hydrothermal systems breccias in the rusey fault cornwall uk and the roamane fault porgera gold deposit papua new guinea provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid these faults contain cockade like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals at rusey the overgrowths are quartz dominant whereas in the roamane fault overgrowths are composed of calcite quartz or gold rich pyrite although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours the spheroidal overgrowths do contact each other and are at least partially cemented together the hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment or finer grained inequigranular to mesh like intergrowths concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles the breccias occur as fault parallel layers and lenses each up to several tens of centimeters thick adjacent layers are characterized by texturally distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths in the rusey fault many texturally distinct breccia layers are present within a 3m wide fault core some layers truncate others and many breccia layers exhibit grainsize grading or banding clasts in the breccias include fragments of wall rock veins and various fault damage products including fragments of earlier generations of cemented breccia brecciation was episodic and separated by periods of cementation the distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow particle size distributions indicate that fluid velocities during fluidization were in the range 0 1 ms 1 to 1 ms 1 the maximum flow rates correspond to fluid fluxes of 10 to 100 l s 1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide such high flow rates characteristically induce intense swarm seismicity rather than mainshock aftershock seismicity and have implications for the dynamics and rates of formation of fault related hydrothermal ore deposits

a study of horizontal to vertical component spectral ratio in the new madrid seismic zone the horizontal to vertical component h v spectral ratio of the small and moderate earthquake ground motions for the shear wave window was used as an estimation of the site response in the new madrid seismic zone nmsz the database used in this study consisted of 500 broadband seismograms from 63 events of magnitude mw 2 5 to 5 2 recorded on 11 stations operated by the university of memphis center for earthquake research and information ceri at the university of memphis all the broadband stations were located within the mississippi embayment soil deposits overlying the rock basement of the embayment strongly affected the amplitudes of the ground motions the horizontal to vertical component ratios were evaluated for the frequency range of 0 2 to 20 hz the observed average h v ratios suggested site amplification between 2 and 4 in the low frequency range f 5 hz for stations located on the lower shear wave velocity deposits lowlands the higher shear wave velocity deposits uplands indicated low frequency amplification be tween 1 5 and 3 hz the observed average h v ratios were also compared with the soil amplifications in the upper mississippi embayment developed by romero and rix 2005 from the 1d equivalent linear method for generic regional profiles the h v ratios were also compared with the theoretical quarter wavelength approx imation these comparisons suggested that the h v ratios could be a first estimate of the site amplifications finally the variability of the h v ratios with distance was examined and no discernible trends were found therefore the path effect model developed by zandieh and pezeshk 2010 for thevertical ground motions in nmsz using the database of this study was also applicable for the horizontal ground motions

silica gel formation during fault slip evidence from the rock record dynamic reduction of fault strength is a key process during earthquake rupture many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments including silica gel lubrication however few have been observed in nature here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface the corona heights fault slickenside in san francisco california is covered by a shiny layer of translucent silica microstructures in this layer show fl ow banding armored clasts and extreme comminution compared to adjacent cataclasites the layer is composed of 100 nm to 1 m grains of quartz hydrous crystalline silica and amorphous silica with 10 100 nm inclusions of fe oxides and ellipsoidal silica colloids kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip the layer therefore represents a relict silica gel that formed during fault motion and which could have resulted in frictional instability these observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions