quartz cementation in late cretaceous mudstones northern north sea changes in rock properties due to dissolution of smectite and precipitation of micro quartz crystals abstract late cretaceous mudstones from two wells located in the northern north sea and the norwegian sea have been examined with respect to quartz cement two different types of quartz cement type 1 and type 2 have been identified using sem eds cl analysis of drill bit cuttings at depths 2370 2670 m 80 85 c type 1 appears as relatively large aggregates 30 100 m of depth temperature related crypto or microcrystalline to macrocrystalline irregular quartz cement formed by local re crystallization of biogenic silica the cl responses of type 1 quartz cement give a clear indication of an authigenic origin type 2 quartz cement represents relatively high amounts of extremely fine grained micro sized 1 3 m crystals embedded as discrete short chains or small clusters nests within the illitized clay matrix the cl responses of micro quartz crystals indicate an authigenic origin the micro quartz is most probably sourced from silica released during the smectite to illite dissolution precipitation reaction the petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones the silica released produce a subtle inter connected micro quartz network interlocked with aggregates of micro quartz and authigenic clay crystals this micro quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime this is indicated by an abrupt increase in well log velocity vp and change in seismic facies close to 2500 m 80 85 c

testing the inference of creep on the northern rodgers creek fault california using ascending and descending persistent scatterer insar data author s jin l funning gj abstract 2017 american geophysical union all rights reserved we revisit the question of whether the rodgers creek fault in northern california is creeping a question with implications for seismic hazard using imagery acquired by envisat between 2003 and 2010 we process two persistent scatterer interferometric synthetic aperture radar insar data sets one from an ascending track and the other from a descending track covering the northernmost segment of the rodgers creek fault between the cities of santa rosa and healdsburg the two different viewing geometries provided by the two different tracks allow us to distinguish vertical velocities which may reflect nontectonic deformation processes from fault parallel velocities which can be used to identify creep by measuring offsets in insar line of sight velocity from 12 fault perpendicular profiles through both data sets we identify seven locations where we have a high degree of confidence that creep is occurring estimated creep rate is more than two standard deviations above zero the preferred creep rates at these locations are in the range 1 9 6 7 mm yr consistent within uncertainty with alignment array measurements creep is probable p 0 70 at another three locations defining a creeping zone 20 km long in total extending northwest from santa rosa we also estimate the map patterns of fault parallel and vertical velocities in the region covered by both data sets these suggest that the rodgers creek fault immediately southeast of santa rosa remains locked

interseismic ground deformation and fault slip rates in the greater san francisco bay area from two decades of space geodetic data author s xu w wu s materna k nadeau r floyd m funning g chaussard e johnson cw murray jr ding x burgmann r abstract 2018 american geophysical union all rights reserved the detailed spatial variations of strain accumulation and creep on major faults in the northern san francisco bay area north bay which are important for seismic potential and evaluation of natural hazards remain poorly understood here we combine interferometric synthetic aperture radar data from the ers 1 2 and envisat satellites between 1992 and 2010 with continuous and campaign gps data to obtain a high spatial and temporal coverage of ground deformation of the north bay the sar data from both ascending and descending orbits are combined to separate horizontal and vertical components of the deformation we jointly invert the horizontal component of the mean velocities derived from these data to infer the deep strike slip rates on major locked faults we use the estimated deep rates to simulate the long wavelength deformation due to interseismic elastic strain accumulation along these locked faults after removing the long wavelength signal from the insar horizontal mean velocity field we estimate fault parallel surface creep rates of up to 2 mm year along the central section of the rodgers creek fault and surface creep rates ranging between 2 and 4 mm year along the concord fault no surface creep is geodetically resolved along the west napa and green valley fault zones we identified characteristically repeating earthquakes on the rodgers creek fault the west napa fault the green valley fault and the concord fault nontectonic deformation in the geysers geothermal field and in late cenozoic basins rohnert park and sonoma basins are also observed likely due to hydrological and sediment compaction processes respectively

creep on the rodgers creek fault northern san francisco bay area from a 10 year ps insar dataset 1 deformation in the northern san francisco bay area is dominated by a series of sub parallel strike slip faults existing gps observations provide some constraint on the slip rates of these faults however these have only limited resolution for resolving shallow fault behavior such as brittle creep we use a 30 image permanent scatterer insar ps insar dataset spanning the time interval 1992 2001 to dramatically increase the density of surface deformation observations we find a discontinuity in observed surface velocities across the rodgers creek fault around santa rosa and further north consistent with shallow creep at rates of up to 6 mm yr the creeping segments are located in areas of local transtension suggesting that lowered normal stresses may play a role in the distribution of creep the existence of creep could significantly reduce expected moment release in future earthquakes on the rodgers creek fault and thus has implications for seismic hazard assessment

the earthquake cycle in the san francisco bay region a d 1600 2012 abstract stress changes produced by the 1906 san francisco earthquake had a profound effect on the seismicity of the san francisco bay region sfbr dramatically reducing it in the twentieth century whether the sfbr is still within or has emerged from this seismic quiescence is an issue of debate with implications for earthquake mechanics and seismic hazards historically the sfbr has not experienced one complete earthquake cycle i e the accumulation of stress its release primarily as coseismic slip during surface faulting earthquakes its re accumulation in the interval following and its subsequent rerelease the historical record of earthquake occurrence in the sfbr appears to be complete at about m 5 5 back to 1850 bakun 1999 for large events the record may be complete back to 1776 which represents about half a cycle paleoseismic data provide a more complete view of the most recent pre 1906 sfbr earthquake cycle extending it back to about 1600 using these we have developed estimates of magnitude and seismic moment for alternative sequences of surface faulting paleoearthquakes occurring between 1600 and 1776 on the region s major faults from these we calculate seismic moment and moment release rates for different time intervals between 1600 and 2012 these show the variability in moment release and suggest that in the sfbr regional plate boundary stress can be released on a single fault in great earthquakes such as that in 1906 and in multiple ruptures distributed on the regional plate boundary fault system on a decadal time scale

amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction there are several known causes of anomalously high porosity in sandstones one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement however there has been no mechanistic understanding of why or how microquartz grows or why it maintains high porosity in sandstones here we have used high resolution scanning electron microscopy electron backscattered diffraction and transmission electron microscopy to study the microquartz cemented late cretaceous heidelberg formation germany we have revealed that a nanofilm of amorphous silica 50 100 nm and a layer of chalcedony are between the detrital grain and microquartz cement the amorphous silica insulates the detrital quartz grains and prevents syntaxial growth while microquartz adopts the orientation of the underlying chalcedony with its fast growth c axis parallel to the grain surface thus preventing growth into the pore now that we know what controls microquartz growth and why it preserves porosity it can be used to help identify rank and appraise deeply buried petroleum accumulations

geochemistry of metalliferous hydrothermal deposits in the aeolian arc tyrrhenian sea abstract in shallow water areas of the submerged volcanic complex around the island of panarea aeolian archipelago hydrothermal precipitation of both low temperature fe oxyhydroxide rich red muds and crusts and high temperature sediment hosted massive sulfides was discovered during an integrated high resolution survey iron rich crusts were also found on the bathymetric high of secca del capo north of salina island the exhalative iron rich sediments occur in small closed depressions or in proximity to faults and scarps at water depths ranging from 55 to 285 m the principal chemical characteristics of these deposits are high but variable fe content ranging from 12 2 to 45 and low contents of the transition elements mn cu zn ni and co the low contents of cu ni and co suggest a hydrothermal origin the fe oxyhydroxide deposits are enriched in light rare earth element ree 35 110 chondrite compared to heavy ree 10 25 chondrite their ree patterns are similar to those of associated calc alkaline volcanics negative slope of light ree and a horizontal distribution of the heavy ones this contrasts with the pattern for iron and manganese oxides of hydrogenous origin which have tilted trends of heavy ree paralleling that of seawater the mineralogy of the polymetallic sulfide deposits consists of galena sphalerite pyrite and barite in the form of silt sand grains and decimeter sized fragments disseminated in clay 30 cm below the seabed at a waterdepth of 80 m the chemistry of the aeolian iron rich low temperature deposits and of the high temperature ba rich pb and zn sulfides suggests that they are genetically analogous to respectively the kuroko type iron formation tetsusekiei and the black ore exposed in the miocene hokuroku district of japan

why the 2002 denali fault rupture propagated onto the totschunda fault implications for fault branching and seismic hazards 1 the propagation of the rupture of the mw7 9 denali fault earthquake from the central denali fault onto the totschunda fault has provided a basis for dynamic models of fault branching in which the angle of the regional or local prestress relative to the orientation of the main fault and branch plays a principal role in determining which fault branch is taken geoearthscope lidar and paleoseismic data allow us to map the structure of the denali totschunda fault intersection and evaluate controls of fault branching from a geological perspective lidar data reveal the denali totschunda fault intersection is structurally simple with the two faults directly connected at the branch point 227 2 km east of the 2002 epicenter the 2002 rupture diverges southeast to become the totschunda fault we use paleoseismic data to propose that differences in the accumulated strain on each fault segment which express differences in the elapsed time since the most recent event was one important control of the branching direction we suggest that data on event history slip rate paleo offsets fault geometry and structure and connectivity especially on high slip rate short recurrence interval faults can be used to assess the likelihood of branching and its direction analysis of the denali totschunda fault intersection has implications for evaluating the potential for a rupture to propagate across other types of fault intersections and for characterizing sources of future large earthquakes

cathodoluminescence of quartz from sandstones interpretation of the uv range by determination of trace element distributions and fluid inclusion p t x properties in authigenic quartz quartz overgrowths from keuper sandstones of the paris basin were examined using cathodoluminescence cl microscopy and spectroscopy coupled with a scanning electron microscope sem with the use of standard cold cl equipment it was observed that the emission of authigenic quartz is much less intense than that of detrital quartz grains but a reversal of intensity was observed with scanned cl in the 200 800 nm range the main cl emission band of diagenetic quartz is at 330 340 nm in the uv range with other bands in the visible range the determination of trace element distributions in authigenic quartz by in situ analyses and by sims imagery reveals the correlation of the 330 340 nm emission band with the highest al and li contents the cathodoluminescence emission of diagenetic quartz in the uv range appears to be influenced by the coupled substitution of al and li into the crystal two explanations are suggested 1 ai li or both serve as activators and 2 the incorporation of al and li causes lattice defects that lead to an enhancement of the intrinsic luminescence trace element analyses and fluid inclusion studies revealed that quartz overgrowths precipitated from a fluid that partially originated from an li enriched primary brine derived from eastern triassic evaporites of the paris basin the specific cl emission band in the uv range can be linked to the diagenetic environment the chemistry of the fluids appears to be the essential parameter whereas precipitation temperatures do not influence the occurrence of such emissions

seismic activity of the san francisco bay region moment magnitude m with objective confidence level uncertainties are estimated for felt san francisco bay region earthquakes using bakun and went worth s 1997 analysis strategy for seismic intensity observations the frequency magnitude distribution is well described for m 5 5 events since 1850 by a gutenberg richter relation with a b value of 0 90 the seismic moment rate m yr since 1836 is 2 68 1018 n m yr 95 confidence range 1 29 1018 n m yr to 4 07 1018 n m yr the seismic moment rate since 1850 is nearly the same m yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906 in contrast m yr since 1977 is about equal that in the 56 years before 1906 80 1 14 of the plate motion moment accumulation rate is available for release in earthquakes the historical m yr and the portion of the plate motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty first century high and low rates of future seismic activity are both permissible given the range of possible seismic cycle recurrence times t and the uncertainties in the historical m and in the percentage of plate motion available for release in earthquakes if the historical seismic moment rate is not greater than the estimated 2 68 1018 n m yr and the percentage of the plate motion moment accumulation available for release in earthquakes is not less than the estimated 80 then for all t the rate of seismic moment release from now until the next 1906 sized shock will be comparable to the rate from 1836 to 1905 when m 6 1 2 shocks occurred every 15 to 20 years

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

extensive authigenic quartz overgrowths in the gas bearing haynesville bossier shale usa abstract in sandstone reservoirs despite grain rearrangement during compaction significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation in mudstone reservoirs grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis where quartz overgrowths have been previously reported in post compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties quartz overgrowths reduce reservoir porosity and permeability we present petrographic evidence of two phases of quartz cement in the haynesville bossier shale i grain replacive and ii quartz overgrowths carbonate grain replacement is volumetrically low petrographic evidence indicates three processes in the development of quartz overgrowths mica and pyrite are i engulfed and ii displaced by quartz overgrowth cement the absence of a supportive primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also iii replacive pressure dissolution of detrital quartz silt grains and smectite to illite transformation are likely sources of silica for quartz cement this study is the first to document large scale replacive authigenic quartz overgrowth development within a producing mudstone

origin of quartz cement in the tirrawarra sandstone southern cooper basin south australia abstract quartz cement is the most abundant authigenic mineral in the fluvio deltaic tirrawarra sandstone and plays an important role in controlling reservoir quality quartz cement ranges from 0 to 19 and is controlled by the original sandstone composition petrographic fluid inclusion electron microprobe and cathodoluminescence cl data from the quartz cement indicate multiple stages of cementation at different temperatures and suggest more than one silica source cl observations indicate up to six stages of quartz cement in some samples the stages of quartz cement can be classified into three zones an innermost zone of brown luminescing cement z1 a middle zone of bright blue luminescing cement z2 and an outer zone of brown luminescing cement z3 dead oil or bitumen is trapped b tween z2 and z3 indicating that z3 formed after oil migration commenced measurements of homogenization temperatures from fluid inclusions in quartz overgrowths indicate that quartz cement precipitated over a temperature range of 65 to 130 c z1 quartz cement formed between 65 and 80 c and z2 cement was precipitated between 80 c and 100 c z3 quartz cement formed later at temperatures around 130 c microprobe analysis shows a consistent variation in aluminum between each quartz cement zone the average aluminum content for z1 z2 and z3 is respectively 237 538 and 58 ppm fluid inclusion precipitation temperatures and aluminum content have been used to help identify the silica sources for different zones of cement a likely source of silica for z1 quartz cement is early alteration of feldspar grains the bright blue cl color in the z2 quartz cement is related to its high aluminum content the source of silica for this cement is likely to have been late dissolution of feldspar grains by acidic fluids generated during kerogen maturation late quartz cement z3 has the lowest aluminum content which is similar to that of detrital quartz grains in the tirrawarra sandstone consi ering the temperature of precipitation very low aluminum content and the presence of z3 cement in facies prone to stylolitization the silica source for the cement is likely to have been pressure solution of detrital quartz at stylolites and grain contacts

a new look back at the 1969 santa rosa california earthquakes on 1 october 1969 two damaging earthquakes of richter magnitude ml 5 6 and 5 7 occurred near the city of santa rosa followed by at least 200 aftershocks these are the two largest events to occur in the northern san francisco bay area since the great 1906 san francisco earthquake and its aftershocks the epicentral area coincides with a 1 km right step over from the northwest striking rodgers creek fault to the healdsburg fault zone and a 6 km right step over to the maacama fault the 1969 sequence and its relationship to the healdsburg and rodgers creek faults has taken on greater significance because of the potential for a moment magnitude mw 7 earthquake occurring on the latter possibly in the next several decades we have relocated the five largest events in the 1969 sequence using the largest aftershock recorded by a temporary network operated by the u s geological survey usgs as the master event the locations and focal mechanisms of the principal events in addition to many of the aftershocks are consistent with right lateral strike slip faulting on the northwest striking healdsburg fault an interesting feature of the aftershocks is that they exhibit a cruciform pattern suggesting possible activity on a northeast striking cross fault located within the step over the significance of such a cross structure is that it may act as a barrier to northward rupture from a large earthquake on the rodgers creek fault seismicity since 1969 continues to image an active southern healdsburg fault surrounding a quiescent rupture zone of the 1969 earthquakes and the possible cross fault the rodgers creek fault is generally aseismic during this period

silica diagenesis i solubility controls abstract the diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts the relationship between solubility and surface area or particle size is sufficient to explain simple opal a opal ct quartz transformations suggesting that the role of temperature and other physical parameters may not be as critical as previously thought complexation and adsorption in an impure system when added to the model allow for possibilities of opal a transformation directly to quartz of clay and zeolite formation from an opaline silica source and of variations in silica transformation rates due to presence of impurities in the system

the most recent large earthquake on the rodgers creek fault san francisco bay area the rodgers creek fault rcf is a principal component of the san andreas fault system north of san francisco no evidence appears in the historical record of a large earthquake on the rcf implying that the most recent earthquake mre occurred before 1824 when a franciscan mission was built near the fault at sonoma and probably before 1776 when a mission and presidio were built in san francisco the first appearance of nonnative pollen in the stratigraphic record at the triangle g ranch study site on the south central reach of the rcf confirms that the mre occurred before local settlement and the beginning of livestock grazing chronological modeling of earthquake age using radiocarbon dated charcoal from near the top of a faulted alluvial sequence at the site indicates that the mre occurred no earlier than a d 1690 and most likely occurred after a d 1715 with these age constraints we know that the elapsed time since the mre on the rcf is more than 181 years and less than 315 years and is probably between 229 and 290 years this elapsed time is similar to published recurrence interval estimates of 131 to 370 years preferred value of 230 years and 136 to 345 years mean of 205 years calculated from geologic data and a regional earthquake model respectively importantly then the elapsed time may have reached or exceeded the average recurrence time for the fault the age of the mre on the rcf is similar to the age of prehistoric surface rupture on the northern and southern sections of the hayward fault to the south this suggests possible rupture scenarios that involve simultaneous rupture of the rodgers creek and hayward faults a buried channel is offset 2 2 1 2 0 8 m along one side of a pressure ridge at the triangle g ranch site this provides a minimum estimate of right lateral slip during the mre at this location total slip at the site may be similar to but is probably greater than the 2 0 3 0 2 m measured previously at the nearby beebe ranch site

quantitative refinement of calibrated 14c distributions abstract a new method is presented for using known ordering or other relationships between 14c samples to reduce 14c dating uncertainty the order of sample formation is often known from for example stratigraphic superposition dendrochronology or crosscutting field relations constraints such as a minimum time between dates and limits from historical information are also readily included dendrochronologically calibrated calendric date histograms initially represent each date the method uses bayes theorem and the relational constraints to upweight date ranges in each date distribution consistent with the other date distributions and the constraints and downweight unlikely portions the reweighted date distributions retain all dating possibilities present in the initial calibrated date distributions but each date in the result now reflects the extra information such as ordering supplied through the constraints in addition one may add information incrementally and thus analyze systematically its effect on all the date distributions thus the method can be used to assess the consistency of the quantitative data at hand the bayesian approach also uses the empirical calibrated date distributions directly so information is not lost prematurely by summarized dates to a mean and variance or confidence intervals the approach is illustrated with data from two densely sampled paleoseismic sites on the san andreas fault in southern california an average reduction in 14 c date distribution variance of 59 is achieved using ordering information alone and 85 is achieved by also applying sedimentation rate constraints and historical information

recrystallization in quartz overgrowths quartz overgrowths are among the most volumetrically significant diagenetic phases in sandstones many combined petrographic and geochemical studies of quartz overgrowths rely heavily on the long held underlying assumption of their stability this study of sem cathodoluminescence sem cl imaging of quartz overgrowths from the jurassic khatatba formation of egypt calls this assumption into question these overgrowths preserve spectacular sem cl textures that can be interpreted as recrystallization fabrics there are three major growth zones in the overgrowths q1 to q3 in the first q1 fine sem cl zoning is preserved some with relict fibrous textures and smooth or bulging growth surfaces these textures could indicate initial precipitation as opaline silica chalcedony microquartz or some phase other than coarse quartz overgrowth in q1 there are discordant areas locally showing smooth and scalloped boundaries and filled with quartz with the sem cl character of later growth zones such textures suggest that earlier zones were crosscut by later phases of quartz the smooth and scalloped boundaries and their position in the overall framework of cements and grains are in places inconsistent with simple fracturing but are consistent with dissolution of earlier growth zones followed by filling with later quartz these features suggest that earlier growth zones were unstable and were replaced by coarsely crystalline quartz overgrowth such observations and other empirical and chemical considerations suggest that in some cases quartz overgrowths may result from recrystallization of less stable quartz and silica phases thus any study of sandstone diagenesis focused on quartz overgrowths should take this possibility into account

sources of authigenic quartz in the permian tight sandstones close to gaoqing fault dongying sag bohai bay basin china abstract authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the permian upper shihezi formation p2s sandstones in the gaoqing area dongying sag bohai bay basin china the sandstones are mainly quartzarenite with minor amounts of clay minerals authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth microfracture healing quartz pore filling quartz and microquartz in the sandstones three distinct separate phases of authigenic quartz i ii and iii were distinguished by using cathodoluminescence microscopy cl in the scanned cl images phase i quartz generally appears with light brown luminescence in the 450 670 nm range phase ii quartz is non luminescent in the 420 480 nm range and phase iii quartz appears with dark brown luminescence in the 470 650 nm range phase i quartz originating mainly from pressure solution has a composition similar to that of detrital quartz grains and was formed at approximately 79 3 104 c in the phase ii quartz li na mg2 al3 and ba2 are enriched and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 c the eu of phase ii quartz is up to 1 42 the silica sources for phase ii quartz are dominated by hydrothermal fluids pressure solution feldspar dissolution and illitisation of smectite also provided a small amount of silica in the phase iii quartz all of the aqueous inclusions are almost higher than its maximum burial temperature al3 mg2 and rb are enriched in most of this quartz which was sourced mainly from hydrothermal fluids pressure solution also provided a very small amount of silica the hydrothermal fluids providing the silica source for phases ii and iii quartz migrated into the p2s sandstones along the adjacent gaoqing fault

quantification of quartz cements using combined sem cl and image analysis quartz cement content is often difficult to quantify using conventional petrography developments in equipment and technique reported here show how excellent high contrast images of quartz cements can be produced using combined scanning electron microscopy and cathodoluminescence sem cl in most cases quartz cement can be differentiated readily from detrital quartz image analysis allows the percentage of authigenic quartz to be quantified advantages of this method are 1 it can be readily automated 2 backscatter electron bse and cl images can be compared directly and 3 images can be improved through acquiring and integrating multiple images comparison of results from conventional optical petrography with those from sem cl suggests that the percentages of quartz cement determined by conventional methods are overestimated where quartz cement is present in high proportions and are underestimated where quartz cement is less abundant by adopting a statistically rigorous approach to image collection the abundance of quartz cement can be determined by sem cl with a known accuracy

detailed mapping and rupture implications of the 1 km releasing bend in the rodgers creek fault at santa rosa northern california airborne light detection and ranging lidar topography reveals for the first time the trace of the rodgers creek fault rcf through the center of santa rosa the largest city in the northern san francisco bay area vertical deformation of the santa rosa creek floodplain expresses a composite pull apart basin beneath the urban cover that is part of a broader 1 km wide right releasing bend in the fault high resolution geophysical data illuminate subsurface conditions that may be responsible for the complex pattern of surface faulting as well as for the distribution of seismicity and possibly for creep behavior we identify a dense magnetic basement body bounded by the rcf beneath santa rosa that we interpret as a strong asperity likely part of a larger locked patch of the fault to the south a local increase in frictional resistance associated with the basement body appears to explain 1 distributed fault normal extension above where the rcf intersects the body 2 earthquake activity around the northern end of the body notably the 1969 m l 5 6 and 5 7 events and aftershocks and 3 creep rates on the rcf that are higher to the north of santa rosa than to the south there is a significant probability of a major earthquake on the rcf in the coming decades and earthquakes associated with the proposed asperity have the potential to release seismic energy into the cotati basin beneath santa rosa already known from damaging historical earthquakes to produce amplified ground shaking

clay mineral diagenesis and quartz cementation in mudstones the effects of smectite to illite reaction on rock properties abstract the late cretaceous to early tertiary sequence of the voring and more basins from the norwegian sea has been examined with respect to mineralogy based on 319 cutting samples from five wells a clear relationship between mineralogy and well log data is demonstrated a significant change with respect to velocity porosity and density occurs within the depth interval corresponding to 80 90 c at shallow depths temperatures

implications of trace element composition of syntaxial quartz cements for the geochemical conditions during quartz precipitation in sandstones a petrographic investigation revealed polyphase quartz cementation in the finefrau sandstone upper carboniferous western germany and the solling sandstone lower triassic central germany three different cements could be distinguished in each sandstone based on their cathodoluminescence and trace element composition the first quartz generation is suggested to have been formed during eogenesis due to dissolution and replacement of feldspar the mesogenetic paragenesis comprises two generations of quartz and illite which are accompanied by albite in the solling sandstone sharp luminescence zoning in quartz overgrowths points to distinct episodes of cementation in both sandstones significant amounts of al li and h and traces of ge and b have been detected in the quartz overgrowths the al content of the quartz cements in the finefrau sandstones exceeds that in the quartz cements in the solling sandstone by a factor of five it is suggested that this compositional variation reflects the conditions in the pore water such as temperature and ph the al concentration is generally correlated to the li content with the exception of the latest quartz generation in the finefrau sandstones which is also most enriched in trace elements the ratio of li al varies between 0 11 and 0 25 in the two sandstones the li h ratio which ranges from 0 12 to 0 3 is controlled by the activity ratio of li and h in the pore fluid clay minerals are the most important source for li and high salinities favour the mobilization of li during diagenesis thus a relatively low salinity and low ph are responsible for the low li h ratio in the finefrau sandstone while high salinity and neutral to alkaline ph results in a high li h ratio for the solling sandstone the ge contents are generally near the average of detrital quartz and indicate that pressure dissolution is a major source for quartz cementation different chemical compositions of distinct quartz generations indicate changes in the physico chemical conditions and point to mobilization of silica from different sources for example pressure solution and clay mineral transformations

a record of large earthquakes on the southern hayward fault for the past 500 years the hayward fault a major branch of the right lateral san andreas fault system traverses the densely populated eastern san francisco bay region california we conducted a paleoseismic investigation to better understand the hayward fault9s past earthquake behavior the site is near the south end of tyson9s lagoon a sag pond formed in a right step of the fault in fremont because the hayward fault creeps at the surface we identified paleoseismic events using features that we judge to be unique to ground ruptures or the result of strong ground motion such as the presence of fault scarp colluvial deposits and liquefaction we correlate the most recent event evidence e1 to the historical 1868 m 6 9 earthquake that caused liquefaction in the pond and recognize three additional paleoruptures since a d 1470 110 yr event ages were estimated by chronological modeling which incorporated historical and stratigraphic information and radiocarbon and pollen data modeled mean age and 95 percentile ranges of the three earlier events are a d 1730 1650 1790 yr e2 a d 1630 1530 1740 yr e3 and a d 1470 1360 1580 e4 the ages of these paleoearthquakes yield a mean recurrence of 130 40 yr although the mean recurrence is well determined for the period a d 1470 1868 individual intervals are less well determined e1 e2 140 80 70 yr e2 e3 100 90 100 yr and e3 e4 150 130 110 yr

timing of large earthquakes during the past 500 years along the santa cruz mountains segment of the san andreas fault at mill canyon near watsonville california a paleoseismic investigation across the santa cruz mountains section of the san andreas fault at mill canyon indicates that four surface rupturing earthquakes have occurred there during the past 500 years at this site right lateral fault slip has moved a low shutter ridge across the mouth of the canyon ponding latest holocene sediments these alluvial deposits are deformed along a narrow zone of faulting there is excellent evidence for a 1906 m 7 8 and three earlier earthquakes consisting of well developed fissures scarps and colluvial wedges deformation resulting from the earlier earthquakes is comparable to that from 1906 suggesting they also were large magnitude events the earthquake prior to 1906 occurred either about a d 1750 1711 1770 or a d 1855 1789 1904 depending on assumptions incorporated into two alternative oxcal models if the later age range is correct then the earthquake may have been a historical early to mid nineteenth century earthquake possibly the a d 1838 earthquake both models are viable and there is no way to select one over the other with the available data two earlier earthquakes occurred about a d 1690 1660 1720 and a d 1522 1454 1605 using oxcal recalculation of the age of the reported penultimate earthquake reported from the grizzly flat site located about 10 km northwest of mill canyon indicates it occurred about a d 1105 1545 earlier than any of the past three earthquakes and possibly correlates to the fourth earthquake at mill canyon

quartz types authigenic and detrital in the upper cretaceous eagle ford formation south texas usa abstract lithologic heterogeneity of the eagle ford formation in south texas arises from mixing of extrabasinal grains of siliciclastic composition with intrabasinal grain assemblages composed dominantly of marine carbonate with a lesser component of biosiliceous debris detrital quartz in particular is derived from both extrabasinal and intrabasinal sources posing a challenge for the use of bulk compositional data for mudrock classification extrabasinal detrital quartz supplied along a major axis of siliciclastic influx the woodbine depositional system of east texas is reduced to a minor part of the grain assemblage in south texas petrographic evidence and point count results indicate that around 85 percent of total quartz in these rocks equal to about 12 6 volume percent is authigenic thus significant quantities of authigenic silica are not restricted to siliceous mudrocks but can be found in carbonate rich mudrocks as well formerly opaline skeletons of radiolaria the dominant source of silica for authigenic quartz precipitation are only poorly preserved by replacements including calcite dolomite pyrite and quartz dissolved silica released by dissolution of radiolarians and perhaps also by volcanic glass dissolution is re precipitated in a variety of forms including matrix dispersed microquartz cement fillings within primary intragranular pores and grain replacement of both calcareous and siliceous allochems the mass balance of dissolved silica mobilized from radiolarians and other reactive silicates and the precipitation of authigenic quartz is uncertain because the initial volumes of now dissolved detrital material versus the final volume of authigenic material quartz and other authigenic silicates cannot be determined with accuracy

material flux and porosity changes during sediment diagenesis understanding the changes in porosity that occur during burial of sediments is of importance both in modelling fluid flow in sedimentary basins and for prediction of petroleum reservoir quality when deposited up to 50 of the volume of sands is intergranular pore space porosity generally decreases with depth because of compaction and precipitation of diagenetic minerals but some sandstones retain an anomalously high porosity whether this is due to dissolution and removal of material or to its local redistribution is not clear we originally intended to quantify losses of major chemical components during burial of a complete sedimentary unit we have investigated ten reservoir sandstones of permian to tertiary age from oilfields worldwide these show consistent results and here we present detailed data from four of them we found that the silica content of these sandstones actually increased by 220 to 350 kg m 3 following compaction no statistically significant changes were observed for aluminium potassium or sodium the flux of silica cannot be modelled satisfactorily using currently accepted values of permeability silica solubility and flow rates posing a challenge for existing models of fluid flow and ore emplacement

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

diagenesis of tight sandstone reservoirs in the upper triassic yanchang formation southwestern ordos basin china abstract in order to better predict the petrophysical properties of the tight sandstone reservoirs in the upper triassic yanchang formation ordos basin china the nature of the diagenetic system involved was investigated using a suite of petrographic and geochemical techniques including thin section and x ray diffraction analysis scanning electron microscopy maps mineralogy cathodeluminescence electron probe microanalysis and fluid inclusion analysis on a set of selected tight sandstone samples the sandstones investigated are texturally mature but compositionally immature with an average framework grain composition of q32f41l27 authigenic quartz calcite and chlorite coatings are the major cements while feldspars are partially dissolved two groups of quartz cement are present in the tight sandstone reservoirs smectite to illite conversion provided silica source for the first group of quartz overgrowth qo i while the silica source for the second quartz overgrowth qo ii was mainly originated from feldspar dissolution calcite cements consist of two types namely ca i and ca ii the ca i calcite cement contains no chlorite coatings and have higher concentrations of fe2 and mg2 whereas the ca ii calcite cement developed over the chlorite coatings plagioclase dissolution appears to be always accompanied by calcite cementation while k feldspar albitization forms euhedral albite locally the burial diagenesis processes were in most cases not episodic but occurred as slow adjustments in response to increased burial depths and temperatures in all the studied diagenetic minerals elemental distributions appear to be strictly constrained by the interaction of authigenic minerals involved indicating that the chemical reactions during the diagenesis comply well with the general principle of mass balance the diagenesis in the upper triassic yanchang formation tight sandstones was thus mainly developed in a closed geochemical system where mineral dissolution and precipitation are approximately balanced and diagenetic fluids were not affected by external sources significantly mechanical compaction has played a more important role in destroying the primary porosities of the upper triassic yanchang formation than the cementation

some aspects of the geochemistry provenance and palaeoclimatology of the torridonian of nw scotland geochemical data mostly from mudstones are used in an attempt to investigate some aspects of the palaeoclimatology and provenance of the torridonian succession in nw scotland the basal part of the stoer group which has been interpreted as glaciogenic is thought to have formed in a warm arid setting the presence of oscillation ripples and desiccation cracks in mudstones containing isolated clasts is thought to preclude their interpretation as ice rafted debris some aspects of the major element geochemistry of the stoer group mudstones suggest deposition in an environment with little weathering but chemical weathering is inhibited in both frigid and arid settings and sedimentological evidence favours the latter the stoer group mudstones are enriched in mg ca and na relative to the torridon group samples which are much closer in composition to an average shale estimate paas the cr and ni contents of the stoer mudstones are significantly higher than those of the torridon group samples there is however no evidence that enrichment in these elements was due to contamination with volcanic material like that of the stac fada member the scourian basement appears to be an adequate source of the ni and cr in the stoer group sediments th sc ratios are higher in the torridon than in the stoer mudstones this difference is interpreted as indicating that the stoer mudstones were largely derived from local archaean sources but that the torridon group mudstones incorporated materials from a much wider provenance including younger and recycled materials evidence from rare earth elements also provides evidence for different source terrains for the stoer and torridon groups the torridon group mudstones are more weathered have a higher ree content and display a more prominent eu anomaly lower value for eu eu than do the stoer samples these results support evidence from th sc ratios that the torridon group was derived from a more varied recycled and granite rich terrain than the stoer much of which appears to have relatively local provenance the geochemistry of mudstones can shed considerable light on problems of provenance and palaeoclimatology because many useful trace elements tend to be concentrated in such fine grained rocks and some aspects of their major element chemistry may be related to climatic factors

the use of strontium isotopes in determining groundwater mixing and brine fingering in a playa spring zone lake tyrrell australia j hydrol 167 225 239 playas in arid and semi arid regions can be important regional groundwater discharge zones located in the northwestern portion of victoria australia lake tyrrell is a large playa that serves as a major discharge zone of regional groundwater the chemistry of the regional groundwater is unique having salinities of approximately 30 g 1 1 and ph values as low as 2 8 as the regional groundwater is discharged along the western edge of the playa in the spring zone it is evapo concentrated into a dense brine which sinks through the playa sediments macumber s pioneering work on the geohydrology of this region established that this denser reflux brine below the playa surface has evolved from the evaporation of regional groundwaters we have measured the 87sr 86sr ratio of a number of groundwater samples taken at shallow depths along the spring zone at the playa edge and compared them with the regional groundwaters and the reflux brine our data reveal a rather complex pattern of mixing of the various water masses associated with the playa system most probably the result of density driven convective plumes two types of regional groundwater can be identified based on their 87sr 86sr values with groundwater from the south being less radiogenic than that flowing from the north i e 0 7108 vs 0 7142 vertical variations in 87sr 86sr in the regional groundwaters observed in wells adjacent to the spring zone can also be identified along a vertical gradient in the spring zone besides the hydrogeological information provided by these measurements the strontium isotope variations allow us to evaluate the source of strontium to these waters the regional groundwaters south of the lake are very similar in 87sr 86sr ratio to the reflux brines whereas the northern regional groundwaters are comparatively enriched in 87sr this enhanced 87sr signature indicates the addition of sr along the flow path via the weathering of the aquifer material the miocene parilla sand the southern waters have 87sr 86sr values closer to that of present day seawater and or miocene seawater indicating that the major source of sr to the system is from one or both of these sources our results demonstrate that strontium isotopes can be extremely useful in studying the dynamics as well as the geochemistry of surface water groundwater interactions

timing of large earthquakes since a d 800 on the mission creek strand of the san andreas fault zone at thousand palms oasis near palm springs california paleoseismic investigations across the mission creek strand of the san andreas fault at thousand palms oasis indicate that four and probably five surface rupturing earthquakes occurred during the past 1200 years calendar age estimates for these earthquakes are based on a chronological model that incorporates radiocarbon dates from 18 in situ burn layers and stratigraphic ordering constraints these five earthquakes occurred in about a d 825 770 890 mean 95 range a d 982 840 1150 a d 1231 1170 1290 a d 1502 1450 1555 and after a date in the range of a d 1520 1680 the most recent surface rupturing earthquake at thousand palms is likely the same as the a d 1676 35 event at indio reported by sieh and williams 1990 each of the past five earthquakes recorded on the san andreas fault in the coachella valley strongly overlaps in time with an event at the wrightwood paleoseismic site about 120 km northwest of thousand palms oasis correlation of events between these two sites suggests that at least the southernmost 200 km of the san andreas fault zone may have ruptured in each earthquake the average repeat time for surface rupturing earthquakes on the san andreas fault in the coachella valley is 215 25 years whereas the elapsed time since the most recent event is 326 35 years this suggests the southernmost san andreas fault zone likely is very near failure the thousand palms oasis site is underlain by a series of six channels cut and filled since about a d 800 that cross the fault at high angles a channel margin about 900 years old is offset right laterally 2 0 0 5 m indicating a slip rate of 4 2 mm yr this slip rate is low relative to geodetic and other geologic slip rate estimates 26 2 mm yr and about 23 35 mm yr respectively on the southernmost san andreas fault zone possibly because 1 the site is located in a small step over in the fault trace and so the rate is not be representative of the mission creek fault 2 slip is partitioned northward from the san andreas fault and into the eastern california shear zone and or 3 slip is partitioned onto the banning strand of the san andreas fault zone

burial diagenesis of illite smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones several gram size samples of 0 1 mu m nearly monomineralic mixed layer illite smectite i s were isolated from nineteen whole cores of shale of the frio formation from the texas gulf coast between approximately 7 000 ft 2133 m and approximately 15 000 ft 4572 m burial depth i s changes from random interstratification r 0 with low 1 to ordered interstratification r 3 with high i the accompanying chemical changes are loss of si mg and fe sup 3 gain of k al and fe sup 2 and a shift in the ree content of the mineral there is no chemical or mineralogical evidence for a cessation of i s reaction at approximately 80 i as is commonly accepted both chemical and mineralogical changes occur throughout burial continuous change in the oxygen isotopic composition of i s implies that the conversion of smectite layers to illite layers is a dissolution and reprecipitation process mass balance calculations indicate that burial diagenesis of frio formation shales is an open system process that requires addition of k sub 2 o and al sub 2 o sub 3 and results in loss of sio sub 2 the amount of sio sub 2 made available by shale diagenesis is sufficient to be the source of the quartz overgrowth cements in the associated frio sandstones iron reduction in i s is amore significant source of the acid required for the diagenesis of both frio shales and sandstones 68 refs 12 figs 4 tabs less

recent rupture history of the san andreas fault southeast of cholame in the northern carrizo plain california we conducted a paleoseismic study on the san andreas fault saf southeast of cholame california to investigate the record of earthquakes along an 80 km paleoseismic data gap between parkfield and the carrizo plain at the ly4 site located 37 5 km southeast of highway 46 along the saf we excavated a fault perpendicular trench on the distal end of an alluvial fan that emanates from the temblor mountains to the northeast we found evidence of three and possibly four ruptures recorded within the stratigraphy the only age constraints are radiocarbon dates on a paleosol three units 50 cm below the oldest event horizon and the presence of recently introduced exotic pollen species in an upper unit the radiocarbon dates indicate there have been at least three surface rupturing events at the ly4 site since cal a d 1058 1291 exotic historic pollen in the top of a unit possibly cut by the youngest event suggests that an earthquake affected at the ly4 site close to a d 1873 1874 manuscript received 11 january 2000

association of earthquakes and faults in the san francisco bay area using bayesian inference bayesian inference provides a method to use seismic intensity data or instrumental locations together with geologic and seismologic data to make quan titative estimates of the probabilities that specific past earthquakes are associated with specific faults probability density functions are constructed for the location of each earthquake and these are combined with prior probabilities through bayes theorem to estimate the probability that an earthquake is associated with a specific fault results using this method are presented here for large preinstrumental his torical earthquakes and for recent earthquakes with instrumental locations in the san francisco bay region the probabilities for individual earthquakes can be summed to construct a probabilistic frequency magnitude relationship for a fault segment other applications of the technique include the estimation of the probability of back ground earthquakes that is earthquakes not associated with known or considered faults and the estimation of the fraction of the total seismic moment associated with earthquakes less than the characteristic magnitude results for the san francisco bay region suggest that potentially damaging earthquakes with magnitudes less than the characteristic magnitudes should be expected comparisons of earthquake locations and the surface traces of active faults as determined from geologic data show sig nificant disparities indicating that a complete understanding of the relationship be tween earthquakes and faults remains elusive

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

significance of trace elements in syntaxial quartz cement haushi group sandstones sultanate of oman abstract trace element contents and distributions in authigenic quartz cement in deeply buried 2500 4000 m haushi group sandstones from wells in oman have been investigated in order to determine the factors that control trace element uptake during precipitation scanning electron microscope cathodoluminescence images show well developed growth zones within the quartz cement which correlate with chemical zonations observed in electron microprobe al distribution maps the most abundant trace elements are al 50 3000 g g 1 li 1 100 g g 1 na 1 40 g g 1 and ge 0 3 5 g g 1 with a strong linear correlation between li and al and a weaker one between ge and al the molar concentration of li na accounts only for 15 of the charge compensation for al 3 substitution of si 4 though h was not measured in this study these data indicate a major role of h in charge balancing al 3 the samples belong to the same stratigraphic unit and have similar petrography but show considerable variability in absolute trace element concentrations between different wells this variability does not correlate with either sample depth or temperature and shows no regional pattern but seems to reflect petrophysical and tectonic differences within the sedimentary basin petrographic observations of the cogenetic mineral assemblages and hydrochemical modelling indicate that a change from the equilibrium assemblage quartz kaolinite dolomite to quartz illite dolomite reflects a decrease in the co 2 concentration and concurrent variations of the al concentration it is concluded that changes in the co 2 concentrations are responsible for fluctuations in fluid al concentrations and thus likely also in the investigated quartz cements

re evaluation of the 1836 hayward fault and the 1838 san andreas fault earthquakes abstract current seismic hazard models include two major earthquakes m 7 in the san francisco bay area that are close in space and time an 1836 event on the northern hayward fault and an 1838 event on the peninsula section of the san andreas fault analysis and interpretation of the available historical accounts indicate that the 1836 event occurred east of monterey bay far from the hayward fault and was of m 6 also the 1838 event was not confined to the 60 km peninsula san andreas as current models indicate instead faulting probably extended from san francisco to san juan bautista 140 km indicating a significantly larger earthquake m 7 than previously thought damaging effects of the 1836 earthquake were reported only from santa clara to carmel and no contemporary effects were reported to the north of santa clara or near the hayward fault the illusion of an 1836 hayward earthquake evolved from a newspaper reminiscence published following the 1868 hayward earthquake stating that the 1868 effects in the east bay were similar to those of an 1836 event the article describes various strong effects in the east bay that differ completely from the effects recorded for the 1836 earthquake but are very similar to those documented for the major 1838 san andreas earthquake that caused extensive damage on both sides of san francisco bay based on this and other evidence we conclude that the reminiscence describes the destructive june 1838 effects but it erroneously indicates the date as june 1836 there is no evidence for any major historical earthquakes in the san francisco bay area before the 1838 earthquake back to the founding of mission san francisco dolores in 1776 during the 1838 san andreas fault earthquake the shaking intensity in monterey was as strong as or stronger than during the great 1906 san andreas fault earthquake this suggests that the 1838 san andreas fault rupture may have extended to san juan bautista as it did in 1906 numerous probable aftershocks were felt in the area south of san juan bautista these damaged carmel and santa cruz in 1840 and alisal 16 km west of the san andreas fault in 1841 the northern end of the 1838 faulting was previously assumed to be 25 km south of san francisco however mission san francisco dolores was damaged in 1838 but not in 1906 suggesting that the 1838 faulting extended to san francisco also the 1838 aftershocks were felt in oakland as frequently and violently as those following the major 1868 hayward earthquake suggesting that the 1838 faulting on the san andreas extended to the latitude of oakland the 1838 fault segment ruptured again 68 years later as part of the overlapping 1906 san andreas fault rupture this and similar evidence from southern california indicates that m 7 san andreas fault earthquakes can recur at intervals of 68 years or less when they are followed by m 8 earthquakes on overlapping segments of the fault

albitization of detrital plagioclase in triassic reservoir sandstones from the snorre field norwegian north sea abstract detrital plagioclase an12 an28 in triassic reservoir sandstones of the lunde formation in the snorre field offshore norway has been partially to completely albitized during burial diagenesis depths of about 2 500 3 000 m 75 100 c present temperature calcite caco3 99 mole and kaolinite were formed as by products of plagioclase albitization sodium rich detrital plagioclase an2 an11 are usually unalbitized detrital k feldspar is fresh or rarely partly albitized differences in albitization behavior of detrital plagioclase and k feldspar are shown to be related to the chemistry of formation waters reactivity of kaolinite and temperature the diagenetically formed albite pseudomorphs are very pure ab 9 well ordered low albite dark luminescent vacuolized mostly untwinned and comprised of numerous euhedral albite crystals some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase the amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable ca rich plagioclase the microporosity formed during albitization is proportional to the anorthite component which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite the albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of na from pore solutions

new evidence for the origin of quartz cements in hydrocarbon reservoirs revealed by oxygen isotope microanalyses quartz cements degrade hydrocarbon reservoir quality by choking off fluid flow paths therefore it is important to understand the sources and timing of quartz cementation we present in situ microanalyses of oxygen isotope ratios in quartz cements from a jurassic hydrocarbon reservoir located offshore norway showing systematic variations that reveal the isotopic history of the pore fluids that precipitated the quartz earliest quartz cements have 18o values of 20 26 subsequent cements drop in 18o value as low as 13 but then increase back to 23 with increasing cement thickness this isotopic trend was observed in multiple overgrowths with temperatures constrained by fluid inclusions and the burial history of the reservoir the isotopic composition of porewaters can be estimated from the measured isotopic ratios in quartz cement results indicate that 97 of the quartz cement precipitated from negative 18o waters originally meteoric that were gradually modified by burial diagenetic reactions as temperatures increased precipitation began in the middle to late cretaceous however the rate of quartz precipitation increased dramatically in the last 3 5 ma as the reservoir was rapidly buried the data show that quartz precipitation is a continuous process during burial and that silica is derived locally from silicate reactions occurring within the sandstone reservoir the isotopic data do not record evidence of episodic cementation from convection driven hot basinal fluids

origin of quartz cement in the lower ordovician dongjeom formation korea abstract this study deals with the origin of quartz cement developed in deeply buried mature sandstones intercalated in a thick carbonate sequence the dongjeom formation early ordovician korea is composed of three coarsening upward sequences and is divided into two facies according to depositional conditions with respect to changes in relative sea level transgressive facies and regressive facies the transgressive facies are characterized by calcite cemented allochem bearing sandstones whereas the regressive facies is characterized by non calcareous quartzose to arkosic sandstones quartz overgrowths occupy significant amounts of pore space average 16 in the regressive facies sandstones fluid inclusions reveal that quartz overgrowths precipitated between 110 and 165 c petrographical study and homogenization temperatures indicate that quartz overgrowths precipitated mostly during and after chemical compaction during the late diagenetic stage considering the 18 o values of detrital quartz grains and the volumes of quartz overgrowth the 18 o values of quartz overgrowth are estimated to range from 14 4 to 19 0 smow on the basis of mass balance silica for quartz overgrowths was principally derived from chemical compaction pressure solution of detrital quartz quartz overgrowths in the dongjeom sandstones precipitated from formation waters 2 1 to 2 5 smow that evolved from contemporary meteoric waters by diffusion

trace elements in hydrothermal quartz relationships to cathodoluminescent textures and insights into vein formation high resolution electron microprobe maps show the distribution of ti al ca k and fe among quartz growth zones revealed by scanning electron microscope cathodoluminescence sem cl from 12 hydrothermal ore deposits formed between 100 and 750 c the maps clearly show the relationships between trace elements and cl intensity in quartz among all samples no single trace element consistently correlates with variations in cl intensity however in vein quartz from five porphyry cu mo au deposits cl intensity always correlates positively with ti concentrations suggesting that ti is a cl activator in quartz formed at 400 c ti concentrations in most rutile bearing vein quartz from porphyry copper deposits indicate reasonable formation temperatures of 750 c using the titaniq geothermometer titanium concentrations of 10 ppm in all veins that formed at temperatures 350 c suggest a broad correlation between ti concentrations and temperature of quartz precipitation in quartz from most deposits formed at 2000 ppm but in high temperature quartz al concentrations are consistently in the range of several hundred ppm aluminum concentrations in quartz reflect the al solubility in hydrothermal fluids which is strongly dependent on ph aluminum concentrations in quartz therefore reflect fluctuations in ph that may drive metal sulfide precipitation in hydrothermal systems

petrography fluid inclusion isotope and trace element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones feldspar dissolution quartz cementation and clay cementation are significant diagenetic processes in deep water fan feldspar rich sandstones in the shengtuo area dongying sag east china the timing and origin of these water rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy fluid inclusion microthermometry oxygen isotope microanalysis and trace element microanalysis three distinct separate episodes of quartz overgrowths referred to as q1 q2 and q3 were distinguished by cathodoluminescence microscopy the q1 quartz identified in all porous sandstones from 2500 to 3600 m 8200 to 11 800 ft was formed at approximately 100 c 115 c 212 f 239 f before oil filled the reservoirs the q2 quartz was mainly precipitated at 115 c 130 c 239 f 275 f accompanying or slightly postdating the main phase of oil filling and was identified in samples from 2800 to 3600 m 9200 to 11 800 ft the q3 quartz was only identified in sandstones buried deeper than 3500 m 11 400 ft and was likely precipitated in the quaternary when temperature exceeded 130 c 135 c 266 f 275 f secondary ion mass spectrometry oxygen isotope microanalyses yielded 18ovsmow vienna standard mean ocean water values ranging from 21 42 to 24 35 for q1 quartz from 22 03 to 24 99 for q2 quartz and from 21 72 to 22 91 for q3 quartz a mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones positive 18o water values 0 5 to 4 5 vsmow of quartz forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water the variations of 18o cements and trace element compositions from q1 quartz to q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly further 18o water values of the pore water increased by approximately 2 vsmow after oil filling meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution the detected co2 and hydrocarbons in fluid inclusions in the quartz cements the existence of co2 in hydrocarbon rich natural gas and the pyrobitumen in the feldspar hosted pores suggest that acids derived from organic matter kerogen in source rocks and hydrocarbons in reservoirs probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones the ongoing development of q2 and q3 quartz suggests that quartz precipitation did not cease after oil filling further diagenetic reactions likely have proceeded from water rock interactions to hydrocarbon water rock interactions

hydrothermal dolomite a product of poor definition and imagination abstract the latest dolomite bandwagon is the hydrothermal dolomite model in its present form this bandwagon is doomed or at least very much overstated for at least two reasons 1 there are several definitions of hydrothermal and hardly any author specifies which one s he is using 2 very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition except the worst as presently applied the term hydrothermal dolomite is confusing and or meaningless we suggest to use white s geol soc amer bull 68 1957 1637 time honored definition of hydrothermal as aqueous solutions that are warm or hot relative to its surrounding environment with no genetic implications regarding the fluid source hence a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature regardless of fluid source or drive furthermore this definition does not carry a lower or upper temperature limit even a dolomite formed at 40 c could be hydrothermal by extension dolomites formed at temperatures lower than ambient are not hydrothermal even if they formed at a rather high temperature for example groundwater may penetrate a rock sequence through a highly permeable pathway such that it is heated to 150 c at a depth where the surrounding rock has a temperature of 250 c we suggest to call dolomite formed from this water hydrofrigid dolomite formed in or near thermal equilibrium with the surrounding rocks may be called geothermal furthermore not all saddle dolomite formation requires advection fluid flow to transport mg saddle dolomite can be formed in at least three ways i e from advection local redistribution of older dolomite during stylolitization and as a by product of thermochemical sulfate reduction in a closed or semi closed system only the first and the last of these three possibilities have a chance of being hydrothermal almost all dolomites and dolostones in the western canada sedimentary basin have recently been re interpreted as hydrothermal applying the rationale outlined above reveals however that this basin contains very little hydrothermal dolomite rather most dolomites in this basin and almost all dolostones south of the peace river arch are geothermal and or the proof of a hydrothermal origin has not been made this has important implications beyond the various case studies at hand as attempts to tie dolomitization to orogenic events become moot at least in the southern part of the basin

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

influence of grain size and temperature on intergranular pressure solution quartz cementation and porosity in a quartzose sandstone abstract cathodoluminescence petrography and scanning electron microscopy sem reveal that grain size and temperature significantly influenced intergranular pressure solution quartz cementation and porosity evolution in the quartzose hartshorne sandstone middle pennsylvanian of the arkoma basin among samples from each locality a negative linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution in contrast a generally positive relationship exists between mean grain size and volume of quartz cement although considerable variability exists in this relationship a positive relationship also exists between mean grain size and porosity intergranular pressure solution directly reduced minus cement porosity by causing tighter pack ng of detrital grains it thereby controlled volumes of both cement and primary porosity for this reason little primary porosity is preserved in pressolved finer grained sandstones even though they contain small absolute volumes of quartz cement appreciably more primary porosity may be retained by unpressolved coarser grained sandstones even though they contain larger absolute volumes of quartz cement these relationships also vary with thermal maturity which increases eastward across the basin this thermal maturity trend is the result of elevated temperatures associated with mesozoic rifting and intrusive events in the mississippi embayment among samples of equal grain size more intergranular pressure solution less quartz cement and less porosity are evident in areas of higher thermal maturity as is the case with grain size the influence that temperature exerted on intergranular pressure solution is in turn reflected in volumes of quartz cement and porosity thus primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher in areas of higher thermal maturity more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity this suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones

clay cutans and the origin of illite rim cement an example from the siliciclastic rotliegend sandstone in the dutch southern permian basin

middle permian environmental changes and shale oil potential evidenced by high resolution organic petrology geochemistry and mineral composition of the sediments in the santanghu basin northwest china abstract shales in the middle permian lucaogou formation have been considered as one of the most important unconventional reservoirs in nw china due to their excellent shale oil and tight oil potential based on the data of previous organic geochemical studies the overall evolution of the ecosystem in the lake water conditions and organic matter sources have been outlined here we present the results of the mineralogical and organic petrological examinations and the rock eval pyrolysis on samples of a fine grained mixed carbonate clastic sedimentary succession from the santanghu basin providing new detailed insights into paleoenvironmental evolution and lake level changes in addition shale oil potential was further evaluated on this oil prone shale analytical results from a total of 196 shale samples from the second member of the lucaogou formation collected from one newly drilled core indicate algal microbial sources of organic matter classified by rock eval pyrolysis as type i kerogen samples characterized by lower hydrogen index hi values type ii kerogen contain increased contributions of terrigenous organic matter evidenced by enhanced inertinite and vitrinite contents vertical variations in mineralogical and bulk geochemical composition reflect an evolution from a saline anoxic to a deep freshwater lake two environmental changes subdivided the sequence into three units the boundary between the lower unit i and the middle unit ii is characterized by the drastic decrease of smectite and k feldspar contents and a sharp increase in total organic carbon toc content the upper unit iii is distinguished by high toc s ratios based on variations in petrography and carbon isotopes of hopanes the middle unit ii can be further subdivided into two parts reflecting changes in water level of the lake all the periods of high water level are characterized by higher lamalginite contents 40 vol and higher hi values than those obtained from sediments deposited during periods of shallower water levels deposition in a shallow lake is evidenced by increased dolomite and decreased lamaginite contents lake level fluctuations indicated by mineralogical and geochemical parameters are associated with changes in organic matter sources leading to the great variability in mineral content organic petrography and hydrocarbon generation potential high toc and brittle minerals contents micro nanopores system and large thickness of the shale show that the lucaogou formation holds a significant shale oil potential in areas where oil window maturity is reached

authigenic quartz microfabrics in cretaceous turbidites evidence for silica transformation processes in sandstones abstract lower cretaceous sandstone turbidites in the scapa field north sea contain a complex variety of authigenic quartz microfabrics early diagenetic microfabrics predate burial calcite cements and compaction they comprise abundant grain rimming microquartz intergranular aggregates of microquartz with authigenic illite smectite and localized isopachous cryptocrystalline quartz and chalcedonite fringes these precipitates contain evidence of precursor opal ct in the form of dissolved recrystallized or pseudomorphed lepispheres and rim cements late diagenetic quartz microfabrics are postcompactional and either replace or overgrow burial calcite cements they display a paragenetic trend from microporous fibrous silica quartzine or chalcedonite to equant syntaxial and mosaic meso uartz cements none of these contain evidence for opal ct precursors and they are likely to have precipitated directly from silica rich pore fluids cathodoluminescence examination of calcite cemented sandstone intervals reveals that siliceous sponge spicules were a major depositional component and were the most likely silica source for opal ct and subsequent quartz cementation dissolution of biogenic silica and formation of opal ct began shortly after deposition with replacement of opal ct by quartz during burial of the scapa member to 1 2 km the early diagenetic silica transformations are identical to those recorded from cenozoic porcelanites and cherts thermodynamic models developed for the diagenesis of biogenic cherts can therefore be applied to the scapa sandstones and qualitatively explain the development of contrasting quartz microfabrics on a bed to bed or smaller scale in contrast direct precipitation of chalce onic quartz during late diagenesis required a resurgence of high supersaturations this is difficult to account for without some import of silica to the sandstones and or cooling of pore fluids precipitation most probably occurred from allochthonous deep basinal brines associated with early stages of hydrocarbon migration

evidence for surface rupture in 1868 on the hayward fault in north oakland and major rupturing in prehistoric earthquakes wgcep90 estimated the hayward fault to have a high probability 0 45 in 30 yr of producing a future m7 bay area earthquake this was based on a generic recurrence time and an unverified segmentation model because there were few direct observations for the southern fault and none for the northern hayward fault to better constrain recurrence and segmentation of the northern hayward fault we trenched in north oakland unexpectedly we observed evidence of surface rupture probably from the m7 1868 earthquake this extends the limit of that surface rupture 13 km north of the segmentation boundary used in the wgcep90 model and forces serious re evaluation of the current two segment paradigm although we found that major prehistoric ruptures have occurred here we could not radiocarbon date them however the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated ad 1640 1776

quartz cement and its origin in tight sandstone reservoirs of the cretaceous quantou formation in the southern songliao basin china abstract the tight sandstones of the cretaceous quantou formation are the main exploration target for hydrocarbons in the southern songliao basin authigenic quartz is a significant cementing material in these sandstones significantly reducing porosity and permeability for efficient predicting and extrapolating the petrophysical properties within these tight sandstones the quartz cement and its origin need to be better understood the tight sandstones have been examined by a variety of methods the sandstones are mostly lithic arkoses and feldspathic litharenites compositionally immature with an average framework composition of q43f26l31 which are characterized by abundant volcanic rock fragments mixed layer illite smectite i s ordered interstratified with r 1 and r 3 is the dominating clay mineral in the studied sandstone reservoirs two different types of quartz cementation modes namely quartz grain overgrowth and pore filling authigenic quartz have been identified through petrographic observations cl and sem analysis homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore filling authigenic quartz formed with a continuous process from about 70 c to 130 c sources for quartz cement produced are the conversion of volcanic fragments smectite to illite reaction and pressure solution at micro stylolites potassium for the illitization of smectite has been sourced from k feldspar dissolution and albitization silica sourced from k feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant the internal supplied silica precipitate within a closed system where the transport mechanism is diffusion the quartz cementation can destroy both porosity and permeability but strengthen the rock framework and increase the rock brittleness effectively at the same time

contemporary seismicity active faulting and seismic hazards of the coast ranges between san francisco bay and healdsburg california contemporary seismicity in the northern california coast ranges from san francisco bay to the town of healdsburg and east of the seismically quiescent north coast segment of the san andreas fault is generally confined to the intensely deformed santa rosa block relative to the adjacent sebastopol block much of this activity is concentrated along the rodgers creek healdsburg bennett valley and maacama faults a prominent gap in microseismicity exists along much of the 43 km long rodgers creek fault although paleoseismological evidence suggests that the fault has generated at least three paleoearthquakes of moment magnitude mw 7 or greater during the late holocene budding et al 1991 earthquakes in the region are generally confined to upper crustal depths of less than 10 12 km right lateral strike slip faulting on both northwest and north striking faults is currently the primary mode of regional crustal deformation based on observed focal mechanisms although both reverse faulting especially in the vicinity of the major quaternary faults and normal faulting occur within the region the tectonic stress field in the region is dominated by north northeast to northeast directed compressive stresses although the rodgers creek fault may pose the highest known level of seismic hazard in the region in the next few decades abundant geological evidence and the contemporary seismicity suggest that faults such as the healdsburg bennett valley and maacama faults also pose significant hazards because of their potential to generate mw 6 7 earthquakes

ground motion modeling of hayward fault scenario earthquakes part i construction of the suite of scenarios abstract we construct kinematic earthquake rupture models for a suite of 39 m w 6 6 7 2 scenario earthquakes involving the hayward calaveras and rodgers creek faults we use these rupture models in 3d ground motion simulations as discussed in part ii aagaard et al 2010 to provide detailed estimates of the shaking for each scenario we employ both geophysical constraints and empirical relations to provide realistic variation in the rupture dimensions slip heterogeneity hypocenters rupture speeds and rise times the five rupture lengths include portions of the hayward fault as well as combined rupture of the hayward and rodgers creek faults and the hayward and calaveras faults we vary rupture directivity using multiple hypocenters typically three per rupture length yielding north to south rupture bilateral rupture and south to north rupture for each rupture length and hypocenter we consider multiple random distributions of slip we use two approaches to account for how aseismic creep might reduce coseismic slip for one subset of scenarios we follow the slip predictable approach and reduce the nominal slip in creeping regions according to the creep rate and time since the most recent earthquake whereas for another subset of scenarios we apply a vertical gradient to the nominal slip in creeping regions the rupture models include local variations in rupture speed and use a ray tracing algorithm to propagate the rupture front although we are not attempting to simulate the 1868 hayward fault earthquake in detail a few of the scenarios are designed to have source parameters that might be similar to this historical event

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

slip rate earthquake recurrence and seismogenic potential of the rodgers creek fault zone northern california initial results instrumental seismicity defines a seismic gap along the rodgers creek fault zone rcfz between santa rose and san pablo bay results of a paleoseismicity study within the gap using offset channels in late holocene alluvial deposits as piercing points indicate a minimum slip rate of 2 1 to 5 8 mm yr for the past 1 300 years a preferred range for the maximum recurrence interval of 248 to 679 years and a surface offset of 2 0 3 minus 0 2 m during the most recent event the rcfz has produced past m7 earthquakes and historical seismicity data indicate a minimum elapsed time of 182 years since the most recent earthquake of this size

using surface creep rate to infer fraction locked for sections of the san andreas fault system in northern california from alignment array and gps data surface creep rate observed along five branches of the dextral san andreas fault system in northern california varies considerably from one section to the next indicating that so too may the depth at which the faults are locked we model locking on 29 fault sections using each section s mean long term creep rate and the consensus values of fault width and geologic slip rate surface creep rate observations from 111 short range alignment and trilateration arrays and 48 near fault global positioning system station pairs are used to estimate depth of creep assuming an elastic half space model and adjusting depth of creep iteratively by trial and error to match the creep observations along fault sections fault sections are delineated either by geometric discontinuities between them or by distinctly different creeping behaviors we remove transient rate changes associated with five large m 5 5 regional earthquakes estimates of fraction locked the ratio of moment accumulation rate to loading rate on each section of the fault system provide a uniform means to inform source parameters relevant to seismic hazard assessment from its mean creep rates we infer the main branch the san andreas fault ranges from only 20 10 locked on its central creeping section to 99 100 on the north coast from mean accumulation rates we infer that four urban faults appear to have accumulated enough seismic moment to produce major earthquakes the northern calaveras m 6 8 hayward m 6 8 rodgers creek m 7 1 and green valley m 7 1 the latter three faults are nearing or past their mean recurrence interval online material high resolution fault system map and tables of creep observations and geographic coordinates of fault model

diagenesis porosity evolution and petroleum emplacement in tight gas reservoirs taranaki basin new zealand abstract tight gas reservoirs have become popular targets in petroleum exploration in recent years due largely to the increasing market demand for gas and also to technology advances used in extraction reservoir quality is typically poor due to deep burial diagenesis resulting in significant compaction cementation and illitization however analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality and these results can potentially be used to help predict reservoir quality at other sites the use of integrated analysis of diagenetic and burial history is here shown for the eocene k3e kapuni group reservoir in cardiff 1 taranaki basin new zealand the k3e belongs to a deep gas play that was considered uncommercial in the early 1990 s due to poor reservoir quality associated with extensive authigenic illite potassium argon dating has shown that destruction of the reservoir through illitization occurred in the pliocene associated with a late stage heating and or fluid flow event timing of illite authigenesis in the k3e reservoir postdates the likely circulation of co2 rich fluids that caused reaction of feldspar to form kaolinite and quartz homogenization temperatures from two phase aqueous fluid inclusions within quartz cements together with 1d basin models of co2 expulsion from coaly source rocks indicate that feldspar reactions may have started in the mid miocene the co2 rich fluids are thought to have been generated by thermal decarboxylation of intraformational paleocene eocene coals at the same time oil was being expelled from older cretaceous coals and migrated into the mid late eocene reservoirs evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil bearing fluid inclusions the products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar suggesting a relatively closed diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved petrographic evidence of reservoir sandstones at cardiff 1 demonstrates the presence of some illite free secondary macropores lined by residual oil this local preservation of porosity may be replicating the original distribution of oil saturated pores in the pliocene at the time when authigenic kaolinite underwent reaction to form illite remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c 5 ma the data presented in this study are consistent with redistributed liquid petroleum into up dip traps and represent the potential for a new exploration play the eocene reservoir at cardiff 1 is currently gas charged with the pore system composed of approximately one third large macropores connected by a tortuous microporous network however it is possible that with the currently buoyant gas market and modern hydraulic fracturing techniques it may now be economic to flow gas from the locally large pores identified in the k3e reservoir

structure and mechanics of the hayward rodgers creek fault step over san francisco bay california a dilatational step over between the right lateral hayward and rodgers creek faults lies beneath san pablo bay in the san francisco bay area a key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step over enhancing its maximum possible magnitude if ruptures are terminated at the step over then another important issue is how strain transfers through the step we developed a combined seismic reflection and refraction cross section across south san pablo bay and found that the hayward and rodgers creek faults converge to within 4 km of one another near the surface about 2 km closer than previously thought interpretation of potential field data from san pablo bay indicated a low likelihood of strike slip transfer faults connecting the hayward and rodgers creek faults numerical simulations suggest that it is possible for a rupture to jump across a 4 km fault gap although special stressing conditions are probably required e g harris and day 1993 1999 slip on the hayward and rodgers creek faults is building an extensional pull apart basin that could contain hazardous normal faults we investigated strain in the pull apart using a finite element model and calculated a 0 02 mpa yr differential stressing rate in the step over on a least principal stress orientation nearly parallel to the strike slip faults where they overlap a 1 to 10 mpa stress drop extensional earthquake is expected on normal faults oriented per pendicular to the strike slip faults every 50 500 years the last such earthquake might have been the 1898 m 6 0 6 5 shock in san pablo bay that apparently pro duced a small tsunami historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step over possibly related to the earthquake we used a hydrodynamic model to show that a dip slip mechanism in north san pablo bay is the most likely 1898 rupture scenario to have caused the tsunami while we find no strike slip transfer fault between the hayward and rodgers creek faults a normal fault link could enable through going segmented rupture of both strike slip faults and may pose an independent hazard of m 6 earthquakes like the 1898 event

holocene paleoecology of an estuary on santa rosa island california abstract the middle to late holocene history and early anglo european settlement impacts on santa rosa island california were studied through the analysis of sediments in a small estuarine marsh a 5 4 m long sediment core produced a stratigraphic and pollen record spanning the last 5200 yr three major zones are distinguishable in the core the lowermost zone 5200 to 3250 yr b p represents a time of arid climate with predominantly marine sediment input and high chenopodiaceae and ambrosia pollen values the intermediate zone 3250 yr b p to 1800 a d is characterized by greater fresh water input and high values for asteraceae and cyperaceae pollen and charcoal particles the uppermost zone 1800 a d to present documents the unprecedented erosion sedimentation and vegetation change that resulted from the introduction of large exotic herbivores and exotic plants to the island during anglo european settlement the identification of pollen grains of torrey pine pinus torreyana documents the persistence of this endemic species on the island throughout the middle to late holocene

evidence for two surface ruptures in the past 500 years on the san andreas fault at frazier mountain california we conducted paleoseismic studies in a closed depression along the san andreas fault on the north flank of frazier mountain near frazier park california we recognized two earthquake ruptures in our trench exposure and interpreted the most recent rupture event 1 to represent the historical 1857 earthquake we also exposed evidence of an earlier surface rupture event 2 along an older group of faults that did not rerupture during event 1 radiocarbon dating of the stratigraphy above and below the earlier event constrains its probable age to between a d 1460 and 1600 because we documented continuous unfaulted stratigraphy between the earlier event horizon and the youngest event horizon in the portion of the fault zone exposed we infer event 2 to be the penultimate event we observed no direct evidence of an 1812 earthquake in our exposures however we cannot preclude the presence of this event at our site due to limited age control in the upper part of the section and the possibility of other fault strands beyond the limits of our exposures based on over lapping age ranges event 2 at frazier mountain may correlate with event b at the bidart fan site in the carrizo plain to the northwest and events v and w4 at pallett creek and wrightwood respectively to the southeast if the events recognized at these multiple sites resulted from the same surface rupture then it appears that the san andreas fault has repeatedly failed in large ruptures similar in extent to 1857

diagenetic modeling to assess the relative timing of quartz cementation and brittle grain processes during compaction this study describes porosity reduction by brittle deformation and the application of touchstone sandstone diagenesis modeling software to assess the relative timing and interactions between grain fracturing and cement formation during burial compaction two examples from a previous study of compactional fracturing are used the oligocene frio formation gulf of mexico basin and the cambrian mount simon formation illinois basin united states grain fracturing during compaction creates intragranular fracture surfaces that are favorable sites for quartz nucleation compared to external grain surfaces that may bear coatings that inhibit the nucleation and growth of quartz cement thus the progress of brittle fracture processes during diagenesis affects quartz cementation in turn modeling of the quartz cementation process can serve to place fracturing into its proper context in burial history in the mount simon formation the extent of brittle deformation of quartz grains correlates with reconstructed effective stress at the onset of quartz cementation for frio formation samples however the extent of brittle deformation does not correlate well with reconstructed effective stress obtained using a one dimensional basin model that uses compaction disequilibrium as the dominant mechanism for overpressure generation judging from the observed degree of grain fracturing significant fluid overpressures in the frio may not have developed at the shallow depths indicated by our basin models the degree of compactional fracturing in sandstones constitutes observable evidence that can be used to decipher the complexities of pressure history

quartz control of high germanium silicon ratios in geothermal waters germanium silicon ge si ratios in himalayan hot spring fluids show a wide range 4 1000 mmol mol equilibrium calculations with an ideal solid solution model of germanium in quartz yield ge sifluid ratios consistent with data from high temperature reservoirs but do not fit lower temperature observations in part because of kinetic constraints a model of progressive si loss via precipitation of ge poor quartz rayleigh distillation can produce the extreme ge sifluid ratios observed in the himalayan and other systems small variations in si loss mixing with surface waters and or disequilibrium can produce the observed variability in ge sifluid within a given hydrothermal system the high level of si loss 95 required to reach extreme ge sifluid ratios is consistent with reaction path calculations for the evolution of a geothermal fluid from its reservoir to surface temperatures and implies that ge si ratios can be a useful tracer of silica dynamics in hydrothermal systems