experimental determination of dynamic pore throat structure characteristics in a tight gas sandstone formation with consideration of effective stress abstract in this study a robust and pragmatic technique has been developed to experimentally quantify the dynamic characteristics of microscopic pore throat structure with consideration of effective stress in a tight gas sandstone formation more specifically porosity and permeability of highly heterogeneous core samples collected from a tight gas reservoir were measured and then thin section analysis as well as other analyses with the scanning electron microscopy and x ray diffraction techniques were performed to identify and classify the particle composition grain size clay minerals and pore and throat types subsequently pore throat structure such as its connectivity radius and size distributions were determined by conducting pressure controlled mercury injection and rate controlled mercury injection tests respectively in addition the nuclear magnetic resonance nmr technique is employed to examine effect of stress sensitivity on porosity and permeability as well as pore and throat radius under different pore pressures by decreasing the pore pressure from 30 0 to 1 0 mpa and the corresponding effective stress was increased from 0 to 29 0 mpa as such the pores and throats become smaller and the average proportion of volume controlled by nano scale throat with the radius less than 100 nm increased from 48 01 to 48 90 while the average volumetric proportion controlled by micron scale and submicron scale throats was decreased from 46 57 to 46 37 the shrinkage of pores and throats resulted in an average reduction for porosity and absolute permeability from 10 43 to 6 84 and from 0 01237 to 0 00501 md respectively moreover samples with a lower permeability showed more serious permeability damage illustrating that they experience a stronger stress sensitivity than those with a higher permeability

rayleigh waves in quaternary alluvium from explosive sources determination of shear wave velocity and q structure surface wave propagation in unconsolidated quaternary alluvium has been investigated within the valley of the chiusi lake tributary central italy the seismic experiment was originally designed to determine the structural response of an etruscan tomb to a 200 kg dynamite shot planned as part of a refraction survey within the crop dss an italian project for the investigation of deep crustal structures the availability of many recording instruments allowed us to design a more complete experiment and we decided to use surface waves generated by a small explosion 10 kg dynamite to infer the shear wave velocity attenuation model in the alluvium layer we deployed a linear array aligned with the shot point on the river sediments sixteen digital three component recording stations were used the minimum distance from the source was 1 9 km and we chose an interstation separation of 50 m for a total array length of 750 m clear rayleigh waves were recorded along the array and we were able to recognize two propagating modes in the waveforms we calculated the dispersion curves of group velocities using a multiple filter technique dispersion curves of phase velocities were calculated using a p stacking technique the whole procedure was run as follows 1 the inversion of the group velocity dispersion 2 the use of synthetic seismograms for mode recognition 3 the extraction and the inversion of phase velocity values from the p stack performed on the observed waveforms 4 the use of a trial and error approach for q to obtain good synthetic waveforms and 5 the application of a conjugate gradient inversion technique to the anelastic absorption model no inversion was run on dispersion curves values of q of ranging from 2 to 20 and a shear wave velocity in the range 280 400 m sec were found in the holocene alluvium moreover in our modeling we used the relation q q for unconsolidated water saturated sediments our results suggest that surface waves generated by explosions can be successfully used to determine shear wave velocity and attenuation models in soft sediment coverages

loess and floods high resolution multi proxy data of last glacial maximum lgm slackwater deposition in the flinders ranges semi arid south australia abstract terrace remnants of late pleistocene fine grained valley fill deposits silts deeply entrenched by ephemeral traction load streams in arid areas remain a puzzle they have been attributed to a variety of origins ranging from lacustrine to alluvial floodplains we here report a centimetre scale multi proxy study of a 7 m section of silts in the semi arid flinders ranges of south australia which span the lead up to and peak of the last glacial maximum lgm the results of detailed lithostratigraphic mapping high resolution parametric particle size analysis automated mineralogy induced magnetic susceptibility carbon stable isotope geochemistry and a chronostratigraphy based on 27 ams radiocarbon and 6 luminescence ages are discussed in terms of sediment provenance depositional environment and weathering history with the aim of reconstructing the regional palaeo environment the data are consistent with a fluctuating aeolian fluvial interplay dominating the extended lgm environment with a greater impact on the landscape than all combined geomorphic processes since then accordingly weathered slope mantles and loess accessions were eroded and entrained by numerous small and at least a dozen large scale flood events and trapped in an intra montane floodplain extending into brachina gorge upstream of this narrow constriction recurrent backflooding is discussed resulting in a thick sequence of layered to laminated slackwater couplets aggradation and degradation of valley fills appear to be largely controlled by fine sediment supply from the valley slopes replenished by wind blown dust from upwind playa lakes and source bordering dunefields in conclusion this study demonstrates how dust storms and flooding rains can account for pluvial features previously explained by the opposing effects of reduced precipitation and evaporation in the colder more arid glacial landscape of southern australia

regional variation and frequency dependence of qβ in the crust of the united states approximate lg attenuation coefficient values are determined for paths in eastern north america at periods between 2 and 4 sec where no data were previously available these new data together with reported values at a period of 1 sec are consistent with values predicted by a frequency dependentent q of the crust in which q varies as 0 2 the new data are inconsistent with values which would be predicted by models having prominent maxima or minima in q 1 within the period range 1 to 5 sec lg attenuation coefficients were computed for the frequency independent crustal q models of cheng and mitchell 1981 for the basin and range province and colorado plateau of the western united states the predicted values at 1 sec for the colorado plateau and basin and range province are about twice as large and three times as large respectively as those predicted for the eastern united states recently reported values of q for 1 sec lg in those regions are consistent with the attenuation coefficients and q values of lg predicted by the colorado plateau and basin and range province models a model with a small degree of frequency dependence of q would be consistent but is not required by the data

paleomagnetic record of two successive miocene geomagnetic reversals in western crete abstract two geomagnetic reversals r n followed by n r have been recorded in a sequence of miocene marine clays of tortonian messinian age in western crete greece the time span of each transition is found to be of the order of 5000 10 000 years the transitional vgp paths are largely constrained in longitude either near the site longitude r n or opposite to it n r a normalized magnetization intensity record has also been obtained and its variations during reversals are found to be in excellent agreement with the predictions of hoffman s phenomenological model

absolute dating of dedolomitization by means of paleomagnetic techniques paleomagnetic analysis in conjunction with petrographic studies is an approach whereby absolute dates can be placed on diagenetic events hematite is associated with dedolomite in the lower ordovician kindblade formation in south central oklahoma and is interpreted as a by product of the dedolomitization of ferroan dolomite paleomagnetic results date the time of dedolomitization as late pennsylvanian early permian dedolomitization was probably caused by oxidizing fluids with high calcium contents that migrated through the kindblade formation following uplift of the arbuckle mountains

shear wave q structure and its lateral variation in the crust of china and surrounding regions summary we have obtained three layer crustal models of shear wave q q for several surface wave paths in china and peripheral regions using a single station multimode method in which amplitude spectra of fundamental mode and higher mode rayleigh waves computed for known source depths and mechanisms are compared to measured spectra the three layers totaling 60 km in thickness roughly comprise the entire crust in the tibetan plateau and include both the crust and part of the uppermost mantle in other regions where the crust is thinner q in the shallowest layer 10 km thick is lowest about 40 in the western portion of the tibetan plateau and highest about 250 in southeastern china in the middle layer at 10 30 km depth q is lowest 60 80 beneath tibet and the pamir thrust system and highest 120 140 in central china and parts of the sino korean platform uncertainties of q in the deepest layer at 30 60 km depth are much greater than in the upper two layers but available results suggest that q is lowest about 80 under the pamir thrust system and highest about 180 under southern mongolia the densities of event and station coverage although wanting in some regions allow us to develop the first regionalized maps of crustal q variation for continents the maps for depth ranges 0 10 km and 10 30 km generally show good agreement with q results obtained earlier with other phases and good correlation with the tectonics of this active region

diagenesis and burial history modeling of heterogeneous marginal marine to shoreface paleocene glauconitic sandstones taranaki basin new zealand paleocene marginal marine to shoreface glauconitic sandstones f sands of the farewell formation from the maui field in taranaki basin new zealand demonstrate a diagenetic evolution driven by major shifts in acidic pore water composition rate of burial and clay mineral authigenesis mechanical compaction is the principal porosity reducing mechanism during the first 2500 m of burial of the f sands continued mechanical compaction with long grain contacts concavo convex contacts and deformed liable grains are common throughout the f sands late stage flow of dissolved co2 in the pore fluids of the farewell formation is thought to have been generated from thermal decarboxylation of coaly source rocks the circulation of these co2 rich fluids will have dissolved into undersaturated pore fluids and partially catalyzed dissolution of feldspar and quartz producing ions for the precipitation of kaolinite and chlorite timing of the diagenetic reactions as determined using paragenetic observations fluid inclusion analysis and burial history modeling suggests that the quartz cements formed at a late stage 100 c corresponding to 0 7 ma and is consistent with the migration of hydrocarbons and associated co2 into the f sand reservoir significant secondary porosity is generated through the dissolution of feldspar which is preserved due to late stage of occurrence at close to present day maximum burial dissolved solutes in the f sands sandstones are being preferentially precipitated in interbedded and surrounding fine grained heterolithic siltstone to very fine grained sandstone beds leading to enhanced heterogeneity and preservation of secondary porosity this study provides an improved understanding for diagenetic reconstruction of marginal marine to shoreface facies

reply to comment on paleomagnetism of a polarity transition in the lower triassic chugwater formation wyoming by emilio herrero bervera and charles e helsley

apparent q for upper crustal rocks of the central rio grande rift spectra from 130 digitally recorded microearthquakes in the magnitude range 0 9 to 0 3 were used to compute apparent seismic quality factors q for upper crustal rocks within 45 km of socorro new mexico in this study apparent q was defined as a measure of seismic wave attenuation due to intrinsic absorption and scattering and was computed over the frequency range 0 50 hz most of the recorded energy was in the 3 30 hz band it was found that apparent q increases with event distance for the eight stations used in this study this increase was successfully modeled with a varying thickness low q low velocity layer lying above a relatively high q high velocity half space it was found that 1 near surface low q regions lying directly beneath the recording sites vary from 0 3 to 2 0 km in thickness and exhibit qp and qs values less than 50 2 the average qs for the upper crust in the central rio grande rift excluding the near surface low q region is 535 95 confidence interval 374 938 3 the observed qp qs ratio for events recorded at eight stations varied from 0 34 to 1 39 for earthquakes recorded at three stations the qp qs ratio decreased with increasing event distance more for two other stations no systematic increase or decrease in qp qs with distance was evident and for the remaining three stations events were too closely clustered to draw any conclusions these changes in the qp qs ratio with increasing event distance may reflect varying degrees of fluid saturation in upper crustal rocks 4 at least three anomalously low qs regions exist within 20 km southwest of socorro these abnormally low q regions may reflect small magmatic intrusions or high fracture densities at depth and may lie in close proximity to microearthquake swarms at depths of 7 2 10 3 km 63 references 10 figures 4 tables less

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

a lower crustal extension to a midcrustal magma body in the rio grande rift new mexico a prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival repeatedly observed on seismograms recorded in the central rio grande rift from intermediate and deep focus earthquakes at distances near 90 is identified as a probable compressional to shear wave conversion from a lower crustal root of the socorro midcrustal magma body smb this phase is inconsistent with previously determined crustal models containing a simple sill like midcrustal magma body with a negligible conduit system finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19 km deep upper surface of the magma body down into the lower crust of the southwestern portion of the albuquerque belen basin the location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the smb and associated seismicity is occurring via an off axis conduit system located beneath the active margin of the albuquerque belen basin

a new paleomagnetic pole for the neoproterozoic uinta mountain supergroup central rocky mountain states usa abstract hematite cemented strata of the neoproterozoic age 800 750 ma uinta mountain group umg provide a new well defined mid neoproterozoic paleomagnetic pole for laurentia thermal demagnetization results and rock magnetic experiments reveal the presence of two discrete magnetizations in umg rocks both magnetizations are carried by hematite the dominant characteristic remanent magnetization chrm is an east directed or antipode shallow positive to negative inclination magnetization that is typically unblocked over a narrow range of high laboratory temperatures between 660 and 680 c the second magnetization exclusively of normal polarity is north to northeast directed and has a moderate to steep positive inclination and probably represents a recent or modern day viscous remanent magnetization the response of umg samples to thermal and chemical demagnetization comparison with previously obtained data from umg rocks in the area results from rock magnetic experiments and the inferred low latitude of laurentia during umg deposition in the neoproterozoic are all consistent with the interpretation that the west or east directed shallow magnetization is a near primary remanence the presence of dual polarities of the chrm with the west directed remanence inferred to be of normal polarity and the east directed of reverse polarity suggests that magnetization acquisition in the umg rocks took place over a sufficient amount of time to average secular variation of the geomagnetic field the mean paleomagnetic pole derived from the inferred primary magnetizations from umg strata 0 8 n 161 3 e 95 4 6 n 9 sampling localities consisting of 79 sites is fully consistent with recently reported results from rocks of mid neoproterozoic age for southwest laurentia which define a tight counterclockwise loop located in the south pacific in the apparent polar wander path for laurentia during the neoproterozoic

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

effect of low q sediments and crustal q on lg attenuation in the united states q values for lg and its coda exhibit significant lateral variations across the united states some of which correlate with known variations in the thickness of shallow sedimentary layers the possibility that all variations in lg attenuation are caused by changes in the thickness of shallow sediments is investigated by computing multi mode synthetic seismograms for crustal models with various thicknesses of low q sediments using crustal velocity and q models obtained in previous studies and reasonable values for the thicknesses and intrinsic q values of sediments in several regions we find that the variation of lg q values between the appalachian and rocky mountains can easily be produced by varying accumulations of sandstone and shale of mesozoic age and younger by contrast throughout most of the western united states neither the low lg q values which have been observed nor the regional variation of those values can be explained by accumulations of low q sediments instead their explanation appears to require low and laterally varying values of q in the crystalline crust

advances in grain size mineral and pore scale characterization of lithic and clay rich reservoirs clay and lithic rich sandstones are difficult to characterize through uncored well sections in terms of their grain size porosity and mineralogy all of which are required for assessing reservoir quality and production performance this paper presents results from a study through one such interval and shows how a combination of different techniques can be used to better understand rock properties of complex reservoirs thereby helping to reduce reservoir uncertainty in this study mean data from laser grain size analysis are comparable to point counted grain size and both are considered as viable analytical methods automated quantitative evaluation of minerals by scanning electron microscopy qemscan provides a further useful and consistent grain size measurement that can be applied to both core and cuttings samples the qemscan has also proved to be a valuable technique in the mineralogical analysis of sandstones that are lithic clay and feldspar rich eliminating the subjective nature that is inherent with optical analysis results from the studied interval show that porosity measured by conventional core analysis ca and mercury injection capillary pressure micp analysis are generally comparable with log derived total porosity porosity measured from point counting and qemscan techniques is significantly lower than total porosity with the qemscan porosity locally equivalent to log derived effective porosity both point count and qemscan porosities show better correlations with permeability and 0 94 respectively than total porosity values and 0 60 ca and micp respectively suggesting that they might provide a measure of effective porosity in high quality reservoir rocks

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

a rock magnetic study of the lower massive sandstone moenkopi formation triassic gray mountain area arizona this paper describes the results of rock magnetic and petrographic studies of the lower massive sandstone of the moenkopi formation north central arizona which heretofore has been considered to be a reliable magnetostratigraphic unit the rock magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens both of which occur within the lower part of the sandstone and stepwise thermal and selective destructive demagnetization sdd of samples of the sandstone containing black sand laminae composed largely of opaque grains in samples both from the claystone clasts and deformed shale lens two secondary components one of modern normal polarity and one of triassic normal polarity were unambiguously identified and a third crm component of triassic reversed polarity was tentatively identified there is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation respectively sdd by means of which the small scale homogeneity of remanence within samples of the lower massive sandstone containing black sand laminae could be assessed shows that each sample primarily contains a heterogeneous three dimensional mixture of triassic normal and reversed components the triassic normal direction that is characteristic of the lower massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components averaging about 55 to 60 normal and 45 to 40 reversed the local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that on a small scale the remanence was sporadically acquired more or less randomly probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite no primary component of remanence acquired at or soon after deposition could be identified the remanence in the black sand bearing lower massive sandstone is largely carried by the black sand grains which as determined from reflected light petrographic studies are primarily martite 67 and altered ilmenite grains 20 composed of complex intergrowths of hematite and iron titanium minerals these data strongly suggest that these grains deposited initially as unaltered to only moderately altered magnetite and ilmenite subsequently underwent oxidation during which they acquired their remanence

early and late paleozoic remagnetization of the cambrian peerless formation colorado abstract the upper cambrian peerless formation in central colorado contains two secondary components of magnetization the dominant component was removed during initial af demagnetization of specimens whereas a weaker component was removed during subsequent thermal demagnetization irm acquisition experiments suggest that the dominant component has low coercivity whereas the weaker component has high coercivity the latter component has southeasterly and shallow directions and the corresponding pole position is located at 40 0 n 134 5 e the high coercivity blocking temperatures over 600 c and petrographic evidence suggest this component resides in diagenetic hematite the magnetization is interpreted as a crm acquired in the late paleozoic the low coercivity component directions fall on a great circle that passes through the modern field direction and two modes at 107 e 10 and 287 e 10 this component is interpreted as a vector sum of two antipodal crms and a modern vrm the pole position 10 n 150 e suggests acquisition in the early paleozoic and the remanence resides either in maghemite or in diagenetic magnetite

determination of petrographic factors controlling permeability using petrographic image analysis and core data satun field pattani basin gulf of thailand abstract miocene sandstones of the pattani basin gulf of thailand undergo progressive diagenesis with depth permeability varies from the millidarcy to darcy range over the short depth interval of 6000 9000 ft 1828 2748 m with a tendency for deeper samples to be less permeable image analysis of representative thin sections showed that these rocks contain five pore types three of which are strongly related to permeability at shallower depths less than 7500 ft 2285 m the relative abundance of the two largest pore types are major controls on enhanced permeability below 7500 ft 2285 m the median throat sizes of these pore types are halved 10 versus 20 m and permeability is greatly reduced a small intergranular pore type unimportant in the upper section has an increased throat size from about 2 to 6 m below 7500 ft 2285 m physical models of permeability allow the construction of artificial rocks where the contributions to permeability of each pore type are calculated at various porosity values these models can be used to assess the maximum permeability likely to be encountered even if such maxima are not observed in core

upper mantle anelasticity and tectonic evolution of the western united states from surface wave attenuation rayleigh wave phase and group velocities and attenuation coefficients in the period range 18 to 120 s have been determined for three regions along nine paths across the western united states using a two station technique the attenuation coefficients were found to increase from east to west between the rocky mountains and the pacific coast rayleigh wave group and phase velocities were inverted using a differential procedure to obtain shear wave velocity models for three regions these velocity models were then used in an inversion process where q 1 as a function of depth was obtained from observations of rayleigh wave attenuation the inversion results show that q values in the upper mantle of the western united states are lowest for the coastal regions and westernmost basin and range and highest for the rocky mountains and western great plains intermediate q values appear to occur in the upper mantle beneath the basin and range province and the columbia and colorado plateaus although uncertainties in the data prevent a clear separation between that region and coastal regions low values of q occur in the upper crust higher values in the lower crust and highest values in the uppermost 20 km of the mantle of all these regions these overlie an upper mantle low q zone in which q values decrease from east to west lowest values 15 20 being similar to those observed above descending slabs in the western pacific the regional pattern of upper mantle shear wave q values is similar to that observed for upper crustal shear wave q values inferred from surface wave studies and for lg coda q values the patterns of upper crustal and upper mantle q variations correlate with the temporal sequence of tectonic and magmatic activity in the western united states lowest q values in both depth ranges occurring where tectonic activity has been most recent partial melting and or enhanced dislocation motion in the mantle and variations in the volume of interstitial hydrothermal fluids in cracks in the upper crust may both be ultimately due to processes which occurred during plate consumption and interaction near the western margin of the united states

multimode rayleigh wave attenuation and qβ in the crust of the barents shelf the multimode method of cheng and mitchell 1981 is used to study the attenuation properties of the crust throughout the barents shelf the sensitivity of the spectra to various source factors and to propagation factors which might characterize a shelf region are thoroughly studied for plane layered models the method is very sensitive to q in the upper crust and to q of sedimentary layers if those layers are thick and highly attenuating it is far less sensitive to q in the lower crust and to q the spectra for shallow strike slip earthquakes are very insensitive to changes in strike slip and dip angles of the fault throughout the entire period range of interest but are very sensitive to variations in focal depth except in the long period range of the fundamental mode and the short period range of the higher modes the fundamental mode but not the higher mode spectra for one path across the barents shelf are apparently distorted at short periods by lateral heterogeneities along the travel path simple two layer trial and error inversions yield a model of the barents shelf with an upper crustal q value of about 80 three layer inversions with a sedimentary layer however yield an upper crustal q of about 250 if the sediments are 1 km thick and are characterized by q values of 40 shear velocity models for the barents shelf show variations between about 3 0 and 3 6 km s in the upper crust along different paths the lower crust shows smaller regional differences but velocities vary between 3 5 and 4 0 km s through the depth range 25 35 km the variation of shear velocities in the upper mantle may correlate with those of the upper crust and vary regionally between about 4 3 and 4 8 km s

pore throat sizes in sandstones tight sandstones and shales pore throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale that continuum is documented in this article using previously published data on the pore and pore throat sizes of conventional reservoir rocks tight gas sandstones and shales for measures of central tendency mean mode median pore throatsizes diameters are generally greater than2 mm in conventionalreservoir rocks range from about 2 to 0 03 mm in tight gas sandstones and rangefrom0 1to0 005 mminshales hydrocarbonmolecules asphaltenes ring structures paraffins and methane form another continuum ranging from 100 a 0 01 mm for asphaltenes to 3 8 a 0 00038 mm for methane the pore throat size continuum provides a useful perspective for considering 1 the emplacement of petroleum in consolidated siliciclastics and 2 fluid flow through fine grained source rocks now being exploited as reservoirs

attenuation of surface waves across the arabian peninsula abstract amplitude spectra of fundamental mode rayleigh and love waves are used to determine shear wave q q models for the upper crust of the arabian peninsula the observed spectra are compared to theoretical spectra computed for earthquakes with known fault plane solutions shear wave q in the upper crust can be obtained by trial and error modeling to fit the the slope of the amplitude spectra at short periods 5 30 s if the source time function is less than about 6 s and if the depth of the earthquake is known to within about 3 km shear velocity q values obtained in this way vary from about 60 along the margin of the red sea to 100 150 in the central part of the peninsula to 65 80 in the eastern folded region these values are unusually low for a stable region and suggest that observed anelasticity there is affected by tectonic and epeirogenic activity which has occurred over the past 30 m y the shear wave q models derived from surface waves at periods of 5 30 s closely predict reported values of lg q at 1 hz shear wave q therefore appears to be independent of frequency in this region at periods between about 1 and 30 s

velocity variations in the crust and upper mantle beneath the tien shan inferred from rayleigh wave dispersion implications for tectonic and dynamic processes we used data from nuclear explosions and earthquakes recorded by the kyrgyzstan telemetered broadband seismic network to examine crustal structure in the tien shan region of central asia seventy nine measurements of fundamental and first higher mode rayleigh waves were analyzed using the single station multiple filtering technique to determine group velocity dispersions we found that there was a distinct difference in rayleigh wave group velocity values between different regions but the data could be combined in three groups 1 paths from the west and south across the turan platform and western tarim basin 2 paths across the eastern tarim basin and 3 nuclear explosions from the lop nor test site group velocity values were inverted to derive one dimensional shear wave velocity models for the tien shan region results reveal crustal thickness varying from 50 to 60 km with thickness increasing toward the eastern tien shan region in agreement with published crustal thickness maps for the region array analysis indicates that paths along the eastern tien shan and tarim tien shan are affected by lateral refractions and multipathing but we argue that these have a small effect on the measured dispersion curves our results indicate that upper mantle velocities in the eastern tarim and tien shan are significantly lower than those beneath the western tien shan and turan platform our results suggest that the boundary between these two regimes lies somewhere west of the depocenter of the tarim basin and east of kashgar china

a back projection method for imaging large scale lateral variations of lg coda q with application to continental africa summary a new method is developed which uses back projection tomography to regionalize large scale lateral variations of coda q for lg waves which have traversed long continental paths successful use of this method requires precise and stable single trace measurements of lg coda q xie nuttli 1988 the method converges rapidly and requires minimal computer storage it also allows quantitative estimation of resolution and error in imaging lateral variations of coda q the spatial resolution of this method is limited by the uneven spatial coverage of the data base by our limited knowledge of lg coda generation and by the trade off between the stability and the spatial resolution of the coda q inversion the method is applied to a large set of digital lg coda data from africa where large scale lateral variations of lg coda q are found to correlate well with major tectonic features most of africa is stable and like other stable regions has relatively high coda q values the lowest values of coda q are associated with the african rift system other regions of low q values include the atlas mountains and cape fold belt regions of mesozoic and younger deformation the lateral variation of frequency dependence of lg coda q correlates in most regions with that of q at 1 hz our analysis of the spatial resolution of this method indicates that the resolving power of lg coda q imaging is comparable to that of velocity tomography using long period surface waves using empirical approaches we estimate that uncertainties in lg coda q and its power law frequency dependence parameter are less than about 60 and about 0 2 respectively for most of africa

attenuation of multiphase surface waves in the basin and range province iii inversion for crustal anelasticity summary a combined inversion forward modelling procedure in which the frequency dependence of shear wave internal friction q 1 is allowed to vary with depth was developed and applied to selected rayleigh wave and lg attenuation data in the basin and range province of the south western united states both q and the frequency dependence of lg waves were used to constrain the models many models can explain rayleigh wave and lg data sets within their uncertainties but at 1 hz most have low values of q 50 80 in the upper 8 km of the crust rapidly increasing values to about 1000 at mid crustal depths and decreasing values at greater depths models which include a layer of higher q values 80 150 in the upper few kilometres of the crust overlying a region of lower q values cannot be precluded by the attenuation data of this study assuming that q varies with frequency as f models for which the frequency dependence is low 0 0 0 1 in the upper crust best explain the data of this study in the lower crust that frequency dependence is not well determined but the models which best explain both the fundamental mode and lg data and produce realistic models of q are characterized at 1 hz by high values of both q and its frequency dependence because of that frequency dependence q may be an order of magnitude lower at a period of 100 s 100 than it is at 1 s 1000 investigations of the effects of changing crustal velocity on values for lg q and its frequency dependence indicate that realistic velocity changes cause only small changes in those values and thus are inconsequential to our results the low q values and their constancy with frequency in the uppermost crust can be explained by fluid flow in a network of cracks in brittle rock increasing q with depth to 10 15 km can be explained by the closing of those cracks due to pressure and their enhanced healing with increasing temperature plastic flow at greater depths may contribute further to the dissipation of cracks and to further increases in q decreasing q values at greater depths can be explained as being the result of increasing temperature increasing content of partial melt enhanced dislocation motion or some combination of these effects

investigation of pore structure and petrophysical property in tight sandstones abstract laboratory measurements include porosity permeability high pressure mercury intrusion hpmi nuclear magnetic resonance nmr measurements and microscopic analysis of thin sections and scanning electron microscope sem were used to gain insight into the nature of the pore throat structure including pore geometry pore size distribution and pore connectivity of bashijiqike tight gas sandstones in keshen 5 keshen 6 keshen 8 keshen 9 and keshen 13 wellblock of kuqa depression the relationships between microscopic pore structure parameters and macroscopic petrophysical behaviors were investigated by regression analysis the results show that various types of pores with wide ranges of pore radius are observed the pore throats are very small commonly rapex controls permeability in tight gas sandstones the nmr pore size distribution is broader than the hpmi pore throat distribution in the nmr analysis only the pore systems connected by the relatively large pore throats should be accounted as movable and some portion of large pores which are not connected by effective large pore throats are not effectively movable the effective movable porosity is calculated by excluding those pores restricted by the tiny pore throats the content of effective movable porosity from nmr measurements shows good correlation relationship with the porosity connected by large pore throats in hpmi analysis three typical types of pore structures are recognized through thin section sem analysis combined with the capillary curve and nmr t2 spectrum and the microscopic and macroscopic characteristics of the three pore structures are investigated the pore structure were comprehensively evaluated and characterized by linking nmr t2 spectrum with hpmi analysis

anelasticity of the crust and upper mantle beneath north and central india from the inversion of observed love and rayleigh wave attenuation data the fundamental mode love and rayleigh waves generated by earthquakes occurring in kashmir nepal himalaya northeast india and burma and recorded at hyderabad new delhi and kodaikanal seismic stations are analysed love and rayleigh wave attenuation coefficients are obtained at time periods of 15 100 seconds using the spectral amplitude of these waves for 23 different paths along northern across burma to new delhi and central across kashmir nepal himalaya and northeast india to hyderabad and kodaikanal india love wave attenuation coefficients are found to vary from 0 0003 to 0 0022 km 1 for northern india and 0 00003 km 1 to 0 00016 km 1 for central india similarly rayleigh wave attenuation coefficients vary from 0 0002 km 1 to 0 0016 km 1 for northern india and 0 00001 km 1 to 0 0009 km 1 for central india backus and gilbert inversion theory is applied to these surface wave attenuation data to obtainq 1 models for the crust and uppermost mantle beneath northern and central india inversion of love and rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20 80 km with lowq for northern india similarly inversion of love and rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 km the inferred lowq value at mid crustal depth high attenuating zone in the model for northern india can be by underthrusting of the indian plate beneath the eurasian plate which has caused a low velocity zone at this shallow depth the gradual increase ofq 1 from shallow to deeper depth shows that the lithosphere asthenosphere boundary is not sharply defined beneath central india but rather it represents a gradual transformation which starts beneath the uppermost mantle the lithospheric thickness is 100 km beneath central india and below that the asthenosphere shows higher attenuation a factor of about two greater than that in the lithosphere the very lowq can be explained by changes in the chemical constitution taking place in the uppermost mantle

petrologic paleomagnetic and structural evidence of a paleozoic rift system in oklahoma new mexico colorado and utah a west northwest trending dike swarm consisting of at least 50 mafic dikes cuts precambrian basement rocks in the area in and south of the black canyon of the gunnison colorado and in unaweap canyon near the colorado utah border the dikes have been studied petrographically petrochemically and paleomagnetically and they have been radiometrically dated lithologically the dike rocks are very similar consisting of tholeiitic diabase composed primarily of labradorite augite and granophyric intergrowths of quartz and alkali feldspar a rb sr isochron based on mineral separates from one dike yields an age of 495 15 m y which is statistically identical with a previously reported rb sr date pooling the age data for these two dikes gives an age of 497 16 m y whole rock k ar dates on four of the dikes are scattered around the rb sr date and indicate that the dikes have been relatively unaffected by reheating or hydrothermal activity since emplacement paleomagnetic directional data from nine of the dikes are well grouped and consistent after both alternating field af and thermal demagnetization several lines of evidence indicate that the remanence reflects the dipolar geomagnetic field direction at the time the dikes were emplaced paleopoles corresponding to the mean directions are located at 37 0 n 101 0 e p 8 6 m 16 4 af demagnetization and 37 0 n 102 3 e p 4 9 m 9 4 thermal demagnetization the consistency of the paleomagnetic data corroborates the idea that the dike swarm is the result of one short pulse of mantle derived magmatic activity the dike swarm and other cambro ordovician igneous rocks in colorado powderhorn alkalic complex wet mountains alkalic complexes and dikes are aligned along a linear west northwest trend cambrian diabase dikes in northeastern new mexico and a bimodal suite basalt gabbro rhyolite granite in southern oklahoma wichita and arbuckle mountains anadarko basin are on line with this trend petrologically all of these cambrian and cambro ordovician igneous rocks represent types that are typically associated with extensional tectonic regimes from these data and consideration of documented paleozoic tectonic activity along the trend of the plutons it is concluded that throughout the paleozoic a west northwest trending tectonic zone extended from southeastern oklahoma through northeastern new mexico and colorado and into southeastern utah bimodal basalt rhyolite and alkalic igneous activity was restricted to the cambrian and cambro ordovician but tectonic activity occurred intermittently along the zone throughout the paleozoic

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

frequency dependence of qlg and its relation to crustal anelasticity in the basin and range province the high observed frequency dependence of q for 1 hz lg waves f0 4 0 6 in the basin and range province can be explained by a frequency independent layered model of intrinsic q for shear waves q which has very low values in the upper crust and rapidly increasing values at greater depths by contrast earlier work in the eastern united states indicated q values in the upper crust which are high and which vary with frequency the q model which best explains reported values of qlg and its frequency dependence in the basin and range consists of an 8 km thick low q layer 60 or less overlying q values as high as 1000 or more at mid crustal depths this q distribution is best explained by an upper crust which contains fluids in interconnected cracks overlying a lower crust where higher lithostatic pressure has closed those cracks these results indicate that whereas the frequency dependence of q is an intrinsic property of crustal material the frequency dependence of qlg arises from both the intrinsic frequency dependence of crustal material and its layered structure a notation is suggested which distinguishes between the frequency dependence of qlg and q

comment on paleomagnetism of a polarity transition in the lower triassic chugwater formation wyoming by emilio herrero bervera and charles e helsley

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

a lower ordovician paleomagnetic pole from the oneota dolomite upper mississippi river valley of 149 oriented specimens of the lower ordovician oneota dolomite subjected to stepwise thermal or alternating field demagnetization 50 contained a characteristic shallow east southeasterly or west northwesterly component of magnetization identifiable on orthogonal vector projections for 48 other specimens the demagnetization trajectories defined greats circle paths which intersect close to the characteristic direction the site mean directions for the characteristic component were determined by using both the characteristic directions and great circle data and these yield a formation mean direction obtained from 10 sites with a declination of 105 an inclination of 2 and a precision parameter k of 17 6 95 11 9 the mean of the site virtual poles falls at 10 4 n 166 4 e k 48 0 a95 7 0 which is close to previously published late cambrian paleopoles both polarity groups have predominantly positive inclinations suggesting the presence of an uncleaned downward component whose unblocking temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization we interpret this downward component as a drilling induced remanent magnetization and we hope to have removed its contribution by averaging the dual polarity results multicomponent magnetizations were also present in samples collected from blocks of oneota dolomite incorporated into the overlying new richmond sandstone lower ordovician but only six out of eight samples yielded well defined greats circle paths the best fit intersection of these defines a northerly and steep recent component while the directions of the higher stability components are different for each block due to the small number of samples and imperfect resolution of magnetic directions the conglomerate test is not conclusive nevertheless it suggests that the stable remanence in the oneota dolomite was acquired prior to the deposition of the new richmond sandstone

reversed polarity characteristic magnetizations in the albian contessa section umbrian apennines italy implications for the existence of a mid cretaceous mixed polarity interval paleomagnetic and paleontologic data from the valle della contessa umbrian apennines italy span a 14 m y gap in previous magnetostratigraphic sections and reveal several intervals of reversed characteristic remanent magnetization chrm in limestones and marls of the albian marne a fucoidi rock magnetic data indicate that the reversed chrm is carried by hematite having high unblocking temperatures 625 690 c hematite with these unblocking temperatures also carries a normal polarity chrm in other intervals a potential bias towards the identification of normal polarity is present since at every horizon measured a normal polarity component carried by magnetite and hematite is isolated at lower unblocking temperatures 300 600 c bulk magnetic properties vary throughout the measured section in the portion containing most of the reversed chrm intervals hematite is the dominant carrier of natural remanent magnetization while magnetite is the dominant carrier stratigraphically above and below these changes correspond approximately with lithology the zone of dominant hematite magnetization is marked by reddish bands pigmentary hematite which can be correlated throughout umbria and are thought to record changes in the oxidation state of the seafloor during deposition two models may account for the observed magnetization directions the reversed magnetizations may record a remagnetization which occurred at least 18 m y after deposition while the beds were still flat lying in reversed polarity chron 33r 83 79 ma or later if the reversed chrm represents a remagnetization the process is potentially of great importance since it can produce magnetization patterns which resemble polarity intervals alternatively the reversed magnetizations may have been acquired during intense seafloor oxidation episodes during the mid albian 107 104 ma and may record unrecognized intervals of reversed geomagnetic field polarity milankovitch like bedding cyclicity can be used to tune the sedimentation rate and obtain estimates of the duration of the potential reversed intervals using these estimates two of the potential reversed polarity intervals are of sufficient duration 100 kyr to be recognizable in both detailed stratigraphic sections and marine magnetic anomaly surveys a primary magnetization model predicts that several intervals of reversed magnetization should be found near the boundary of the biticinella breggiensis and ticinella primula foraminiferal zones and within the prediscosphaera cretacea nannofossil zone on a worldwide basis

seismic velocity and q structure of the middle eastern crust and upper mantle from surface wave dispersion and attenuation observed velocities and attenuation of fundamental mode rayleigh waves in the period range 7 82 sec were inverted for shear wave velocity and shear wave q structure in the middle east using a two station method additional information on q structure variation within each region was obtained by studying amplitude spectra of fundamental mode and higher mode rayleigh waves we obtained models for the turkish and iranian plateaus region 1 areas surrounding and including the black and caspian seas region 2 and the arabian peninsula region 3 the effect of continent ocean boundaries and mixed paths in region 2 may lead to unrealistic features in the models obtained there at lower crustal and upper mantle depths shear velocities are similar in all three regions shear velocities vary significantly in the uppermost 10 km of the crust being 3 21 2 85 and 3 39 km s for regions 1 2 and 3 respectively q models obtained from an inversion of interstation attenuation data show that crustal shear wave q is highest in region 3 and lowest in region 1 q s for the upper 10 km of the crust are 63 71 and 201 for regions 1 2 and 3 respectively crustal q s at 30 km depth for the three regions are about 51 71 and 134 the lower crustal q values contrast sharply with results from stable continental regions where shear wave q may reach one thousand or more these low values may indicate that fluids reside in faults cracks and permeable rock at lower crustal as well as upper crustal depths due to convergence and intense deformation at all depths in the middle eastern crust

anelasticity of the crust and upper mantle of south america from the inversion of observed surface wave attenuation fundamental mode rayleigh and love waves generated by several earthquakes situated along great circle paths between pairs of seismograph stations have been analysed to obtain coefficients of attenuation group velocities phase velocities and specific quality factors in the period range 18 80s in two regions of the south american continent one set of paths crosses the shield region which lies on the eastern coast and another set traverses the mountainous region inland the average attenuation coefficient values are clearly higher in the tectonically active western region throughout the entire period range than in the eastern or shield region inversion of the attenuation data yielded shear wave internal friction q 1 models as a function of depth in the crust and upper mantle in both regions a low q zone below the lithosphere is prominent in both regions the results show that substantial variations of q occur in the two regions of south america the q values were found to be inversely related to the heat flow values or to the temperature

measurement of frequency dependent lg attenuation in the great basin measurement of the spatial decay of spectral amplitudes of the higher mode seismic surface wave train lg indicates that in the great basin the apparent seismic quality factor q is a function of frequency observations along a 300 km profile recording of a nuclear explosion yield the function q 206 0 68 over the band 0 3 10 0 hz similar analysis using numerous recordings of earthquakes along paths which provide a good average sampling of the great basin crust gives q 214 15 0 54 0 09 for 0 3 5 0 hz if the crustal sampling by lg energy from nuclear explosions surface sources is primarily in the shallow crust as has been suggested then these results indicate a greater frequency dependence of apparent q there than at depth

anelasticity of the iceland plateau from surface wave analysis we study the anelasticity of the asthenosphere beneath the youngest seafloor of the iceland plateau region using spectral analyses of surface waves the epicenters and station geometry naturally define a fan shooting experiment with pure path propagation in regions with different seafloor ages both rayleigh and love waves are used in this study to help constrain the source parameters needed for a reliable assessment of q the inversion results indicate an approximately 130 km thick layer of q of about 7 for the asthenosphere of the youngest region of 0 5 ma for the region of age 5 10 ma q is about 50 and increases to over a hundred for the oldest zone 10 ma these attenuation values when combined with laboratory experiments indicate a partial melt of 3 in the asthenosphere near the spreading ridge

regional variation of lg coda q in the continental united states and its relation to crustal structure and evolution records from broadband digital stations have allowed us to map regional variations of lg coda q across almost the entire united states using a stacked ratio method we obtained estimates of q 0 lg coda q at 1 hz and its frequency dependence for 218 event station pairs those sets of estimates were inverted using a back projection method to obtain tomographic images showing regional variations of q 0 and q 0 is lowest 250 300 in the california coastal regions and the western part of the basin and range province and highest 650 750 in the northern appalachians and a portion of the central lowlands intermediate values occur in the colorado plateau 300 500 the columbia plateau 300 400 the rocky mountains 450 550 the great plains 500 650 the gulf coastal plain and the southern portion of atlantic coastal plain 400 500 and the portions of the central lowlands surrounding the high q region 500 550 the pattern of q 0 variations suggests that the united states can be divided into two large q provinces one province spans the area from the rocky mountains to the atlantic coast is tectonically stable and exhibits relatively high q 0 the other extends westward from the approximate western margin of the rocky mountains to the pacific coast is tectonically active and exhibits low q 0 the transition from high to low lg coda q in the western united states lies further to the west than does an upper mantle transition for q and electrical resistivity found in earlier studies the difference in q 0 between the western and eastern united states can be attributed to a greater amount of interstitial crustal fluids in the west regions of moderately reduced q within the stable platform often occur where there are accumulations of mesozoic and younger sediments reduced q 0 in the southeastern united states may not be due to anelasticity but may rather be explained by a gradational velocity increase at the crust mantle boundary that causes shear energy to leak into the mantle

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

absolute dating of dedolomitization and the origin of magnetization in the cambrian morgan creek limestone central texas goethite associated with dedolomite in the morgan creek limestone in central texas carries a chemical remanent magnetization crm the pole position 88 n 96 w k 67 95 2 7 which is close to that of the modern pole and the fact that only normal directions are present suggest that the component is a post brunhes 700 000 yr crm dedolomite is common in the morgan creek and consists of calcite rhombohedra with relict dolomite goethite occurs surrounding the rhombs and in rhombic zones within the crystals the goethite precipitated when iron was released from ferroan dolomite during the dedolomitization process the paleomagnetic data therefore date the dedolomitization process as modern dedolomitization is probably related to surface weathering of the morgan creek in addition to goethite associated with dedolomite the morgan creek contains hematite replacing fossils and glauconite and detrital hematite the morgan creek also contains three other magnetic components one of these components is a viscous remanent magnetization aligned with the present magnetic field another component is a triassic jurassic crm residing in hematite a fourth component in the morgan creek is an early paleozoic magnetization residing in hematite and minor magnetite and represents the vector addition of a detrital remanent magnetization and one or more crms the results of this study demonstrate that paleomagnetic and petrographic techniques can be used to date a diagenetic event and also illustrate the complex nature of magnetization in a sedimentary unit

paleomagnetism of a polarity transition in the lower triassic chug water formation wyoming a total of 156 oriented specimens taken at stratigraphic intervals of 3 to 20 cm have been collected from the red peak member of the chugwater formation at three localities approximately 25 km southeast of dubois wyoming the stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1 0 m all specimens were thermally demagnetized at 500 550 620 and 660 c and show directions of magnetization of normal reversed and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone the section sampled is divided into five intensity periods two periods recording the dipolar field one recording a dipolar field of decreasing intensity one recording a dipolar field of increasing intensity and a period of low intensity recording the field present during the 180 directional swing a virtual geomagnetic pole vgp path obtained for a n r polarity transition makes a fairly smooth transit with the path confined to a sector of longitude between 85 w and 110 e the reversal s onset is first indicated by a slow movement of the vgp across northern latitudes the vgp then lingers to two positions 60 n and 40 n 35 e to 70 e and 20 n and 40 n 335 e to 5 e suggesting brief periods of field stability or nondipolar field dominance the onset of the transition appears to be more rapid than the recovery the vgp path observed at a northern hemisphere site is located about 90 from the site the pole position derived from stable normal and reversed portions of this study 115 5 e 46 4 n 95 4 1 dp 2 2 dm 4 2 lies approximately in the center of the main group of poles reported for other triassic units

an application of the combined conglomerate fold test to the study of foreland basins palaeomagnetic results from the southeastern ebro basin summary the triassic limestones of the cover of the catalan coastal range display a secondary magnetization acquired after folding which took place during the palaeogene the exposure and subsequent erosion of these rocks during the palaeogene resulted in the formation of conglomerates with triassic limestone cobbles in the southeastern ebro basin palaeomagnetic data indicate that both the triassic limestones which form the cover and the triassic limestone cobbles from the conglomerate were remagnetized during the same time span thus it seems that fluids were expelled out of the margins into the basin during deformation causing the remagnetization both in the foreland basin environment and in the folded margin

coda q in the eastern caribbean west indies summary coda q q has been estimated from seismograms of local earthquakes recorded on short period seismographs on some of the eastern caribbean islands of the lesser antilles arc west indies using the s s single scattering model the region was subdivided into four distinct subregions in order to investigate lateral variations in q the qc values exhibit a strong frequency dependence in the frequency band 1 5 to 16 hz with calculated coda q at 1 hz qo ranging from a minimum of 97 in the dominica area to a maximum of 145 in the leewards the degree of frequency dependence is also largest in the dominica area n 1 09 and smallest in the leewards n 0 82 although there appears to be some lateral and depth variation of both q and n in the region this variation does not seem to be very significant

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp

anelastic rayleigh wave attenuation in the iberian peninsula summary fundamental mode attenuation coefficients of the vertical components of rayleigh waves have been obtained for three different tectonic regions of the iberian peninsula the western part of iberia which is tectonically most stable has the lowest attenuation and the largest quality factor q average q at depths from 0 to 80 km is 126 84 in the tectonically stable part of western iberia and 57 22 in the active part of eastern iberia

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

crustal q structure in the united states from multi mode surface waves determination of q structures are made using a new method which matches observed surface wave spectra with theoretical spectra computed for earthquakes with known fault plane solutions the importance of the method lies in its ability to utilize relatively short paths between a single source and receiver both of which are preferably located within a single geological province the method has been applied to study the regions of the eastern united states the colorado plateau and the basin and range province q models have been obtained through trial and error procedures and with an inversion scheme developed for that purpose models for each region obtained from these two procedures are in good agreement q values in the upper crust as obtained from inversions are 275 for the eastern united states 160 for the colorado plateau and 85 for the basin and range province q values in the lower crust although not well determined must be much higher than upper crustal values in all three regions the conclusions are consistent with those of previous work in which q values in the upper crust of a broad region of the western united states are lower than those of the eastern united states these new results show that substantial variations of q occur in different regions of the western united states the upper crustal q values show an inverse relationship to heat flow values

petrography and reservoir physics i objective classification of reservoir porosity porosity observed in thin section can be objectively classified using a combination of digital acquisition procedures and pattern recognition algorithms pore types are derived from the frequency distributions of sizes and shapes of patches of porosity exposed in thin section each pore type is represented by a characteristic distribution of sizes and shapes found in thin section most sandstone reservoirs contain fewer than six pore types much of the intersample variability in reservoir physics is associated with changes in pore type abundance the advantages of this approach to porosity classification are 1 the criteria for classification are objectively defined 2 classification procedure is rapid accurate and precise 3 pore types are understood easily in ter s of conventional genetic classification schemes and 4 pore type data are related strongly to petrophysical properties

paleomagnetism of latest cambrian early ordovician and latest cretaceous early tertiary rocks of the florida mountains southwest new mexico paleomagnetic investigations have been carried out on latest cambrian to early ordovician alkaline intrusions their contact intermediate to mafic composition metaigneous rocks and overlying lower paleozoic and early tertiary strata in the florida mountains southwestern new mexico a well defined predominantly normal polarity magnetization of high median destructive induction and high usually discrete laboratory unblocking temperatures is isolated from sites in metaigneous rocks in contact with syenites exposed on the northwestern flank of this late cenozoic uplift combinations of stable endpoint and planar data are used in data analysis at the site level d 262 i 30 95 10 k 12 n 22 site means in addition to the presence of reverse polarity magnetizations at two sites the dispersion of site mean directions in these rocks vgp angular standard deviation of 19 6 suggests that considerable variation of the geomagnetic field has been averaged after correction for a combination of early tertiary laramide deformation and late tertiary basin range block tilting this magnetization gives a paleomagnetic pole at 6 s 169 e and is interpreted to have been acquired soon after intrusion of alkaline plutons at 503 10 ma based on published u pb zircon isotopic age determinations a lack of well defined coherent magnetizations in the plutonic rocks may reflect the complex subsolidus alteration history of these rocks as interpreted with rb sr isotopic data hematitic sandstones of the earliest ordovician bliss formation which directly overlie the crystalline rocks in portions of the range give dual polarity magnetizations whose directions suggest that these rocks have probably been remagnetized this remagnetization possibly occurred during deformation in late cretaceous tertiary time ordovician carbonate rocks of the el paso and montoya formations give only a holocene magnetization paleocene to earliest eocene hematite cemented clastic rocks of the lobo formation possess dual polarity magnetizations the origin of their magnetization is difficult to assess with the available data in total however paleomagnetic data from the bliss and lobo formations argue against any significant vertical axis rotation of the area since laramide deformation ending in early tertiary time the intermediate to mafic composition metaigneous rocks from the florida mountains provide a latest cambrian early ordovician paleomagnetic pole that compares favorably with other north american paleopoles satisfying several acceptance criteria the florida mountains data further support recent constructions of apparent polar wander paths for north america and thus the hypothesis that significant counterclockwise rotation of north america occurred during the ordovician