ironstone bodies of the barberton greenstone belt south africa products of a cenozoic hydrological system not archean hydrothermal vents irregular bodies of goethite and hematite termed ironstone pods in the barberton greenstone belt south africa have been previously interpreted as the earth9s most ancient submarine hydrothermal vent deposits and have yielded putative evidence about archean hydrothermal systems ocean composition and temperature and early life this report summarizes geologic sedimentological and petrographic evidence from three widely separated areas showing that the ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems probably during periods of higher rainfall in the pleistocene the deposits include a recently active spring terrace and outflow system containing iron oxide encrusted microbial filaments shallow subsurface groundwater areas showing ironstone deposited between chert blocks and replacing chert degraded spring and outflow deposits and goethite cemented pleistocene landslide breccias and regolith the predominance of goethite a thermally unstable iron oxide widely developed slope parallel stratification abundant primary open cavities some with vertical goethite dripstone and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict archean vein and cavity fill quartz after iron oxide replacement of surrounding chert iron was sourced by archean sideritic sedimentary units that hold up high ridges on which the ironstone bodies occur siderite was dissolved by circulating groundwater and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the fe rich water flowed onto the surface these deposits represent a remarkable iron oxide depositing quaternary hydrologic system but provide no information about conditions or life on the early earth

neogene tectonic rotation of the santa ynez range western transverse ranges california suggested by paleomagnetic investigation of the monterey formation a paleomagnetic investigation was conducted on dolomite beds in the middle and upper miocene monterey formation in order to determine the timing and amount of post early miocene tectonic rotation experienced by the santa ynez range part of the western transverse ranges province of california five stratigraphic sections of the monterey formation exposed along the coastline west of santa barbara were extensively sampled the sections were correlated using tuff beds and ages were assigned to each dolomite bed on the basis of magnetostratigraphy and diatom biostratigraphy the dolomite beds possess stable primary remanent magnetic directions which pass fold tests and reversal tests and show a consistent decrease in magnetic declination with decreasing age rocks 14 8 to 15 8 m y old have a mean magnetic declination of 92 7 whereas rocks 8 to 11 m y old have a mean magnetic declination of 36 7 the consistent upsection decrease in magnetic declination within these stratigraphic sections is attributed to a rapid 56 10 clockwise rotation of the entire santa ynez range between 15 2 0 1 and 10 1 m y b p the mean magnetic declination of 36 7 in rocks 8 to 11 m y old is probably due to continued clockwise rotation of the santa ynez range during the plio pleistocene an early miocene palinspastic reconstruction is proposed in which the area of the presently east west trending santa ynez range is restored to an initially north south orientation

comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the basin and range province direct comparison of low fold high energy explosive and high fold lower energy vibroseis methods for acquiring deep crustal seismic reflection data in the basin and range province suggests that the high fold common midpoint cmp method there does not provide the best possible image of lower crustal structure during the recent acquisition of a vibroseis profile in the basin and range province we fired single deep shot holes to obtain a coincident single fold explosive section within the upper crust upper 3 s the explosive source and vibroseis records are nearly equivalent for record times below 3 s however comparison of the explosive source gathers and the coincident final 60 fold vibroseis section demonstrates that low fold explosive profiling provides a higher quality image of the midcrust to lower crust 3 10 s the higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives making them less sensitive to common noise sources whereas deeper than 4 5 s the vibroseis energy levels on individual source efforts fall to that of ambient noise levels the explosions provide signal generated energy exceeding ambient noise levels down to 18 19 s although individual reflections can be correlated on explosion and vibroseis shot gathers reflection events on the 60 fold vibroseis stack do not correlate to those on the single fold explosion profile suggesting that the high fold cmp method in our study did not maintain the integrity of the weak lower crustal reflected arrivals reasons why the high fold cmp method apparently failed include complex even time varying statics nonhyperbolic moveout at long offsets and the difficulty in resolving stacking velocities with data having low signal to noise ratios reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower energy vibroseis section

sedimentary geology of the palaeoarchaean buck ridge south africa and kittys gap western australia volcano sedimentary complexes abstract the two palaeoarchaean volcano sedimentary complexes of the buck ridge barberton greenstone belt south africa and kittys gap coppin gap greenstone belt east pilbara australia have a similar geological setting and age 3 45 ga the predominantly volcaniclastic sediments are concentrated at the top of these complexes and experienced thorough very early diagenetic silicification in many places the silicification process has led to excellent preservation of the primary sedimentary structures elsewhere it has resulted in their obliteration or replacement by diagenetic structures the buck ridge chert forms a regressive transgressive succession deposited around base level with lacustrine and littoral marine facies deposition of the kittys gap chert was also close to base level almost exclusively subaqueous with tidal influence and a regressive sequential trend in both volcano sedimentary complexes these low energy sediments are juxtaposed with high energy breccia pods and layers with often a high fe oxide content the breccias are interpreted as being the result of explosive hydrothermal activity sedimentation was strongly controlled by normal faulting

harmonic distortion reduction on seismic vibrators to improve seismic resolution using vibroseis vibrators must output the ground force energy with minimum harmonic distortion four main sources within the vibrator system cause the ground force to suffer severe harmonic distortion the nonlinear servo valve flow pressure character istics the servo valve characteristics near null the fluctuations in the hydraulic pressure and the nonlinear contact stiffness between the baseplate and the ground modeling was undertaken to investigate and quantify harmonic distortion from each source as a result three modifications have been made to reduce the harmonic distortion of the ground force

the effects of inter vibrator interference on vibrator performance the output from the hydraulic vibrators typically used for land seismic surveys is controlled by monitoring the acceleration measured by accelerometers mounted on the reaction mass and baseplate the considerable energy output by such vibrators which are coupled with the sensitivity of the accelerometers used results in crosstalk if more than one vibrator is being used in this paper we present the results of a field experiment in which we measured the crosstalk between two adjacent vibrators we found that the level of crosstalk was approximately 20 db when the vibrators were adjacent but decreased with increasing frequency and separation this result has implications for measurements of vibrator performance source signature deconvolution and in particular estimates of the total energy output by a fleet of vibrators

sedimentation response to earthquake related events middle eocene ventura basin california middle eocene strata in the topatopa mountains northeastern ventura basin california were deposited as five unconformity bounded depositional sequences in a seismically active basin characterized by rapid episodic subsidence analysis of stratigraphic geometries detailed facies analysis and backstripping derived subsidence rates indicate that tectonically induced differential subsidence caused basin wide migrations of the topatopa depocenter at least every 2 m y unusually abundant convolute laminations and a slumped interval may record the influence of earthquakes the convulate laminations do not grade downward into ripple laminations and are not associated with dewatering dikes and pipes as would be expected if the laminations were formed by shear from an overlying current or by dewatering during rapid burial thus formation during shock induced liquefaction is more likely also a 20 m thick slumped interval in the uppermost cozy dell formation underlies a sequence boundary interpreted as a surface across which depocenter migration took place association of this slumped interval with a tectonically formed surface is consistent with deposition in a seismically active environment rapid differential subsidence in the topatopa depocenter was probably episodic and associated with seismic events rapid episodic subsidence with attending earthquakes is recorded in holocene strata of the humboldt basin an analog to the topatopa depocenter also a middle tertiarymore slumped interval in the southernmost san joaquin basin is attributed to a seismic event similar earthquake related events are recorded by sedimentation patterns in the middle eocene ventura basin and may be evident in the strata of other active margin basins as well less

latest cretaceous basin formation within the salinian terrane of west central california sedimentologic details provide new information about the controversial late cretaceous early tertiary history of the salinian terrane a displaced crustal block located in the california coast ranges the preserved upper cretaceous strata were deposited as part of a fan delta submarine fan system along the margin of a topographically steep basin bounded by normal faults coarse grained sediments derived from local basement rocks and volcanic rocks of a mature magmatic arc were shed southward from a basement high north of the basin sediment compositions suggest connections to the southwestern mojave desert region paleomagnetic data collected from the central salinian terrane indicate minimal northward transport relative to cratonal north america and support the hypothesis of an origin in southern california rather than in southern mexico central america the upper cretaceous salinian basin may have formed as an east west trending fore arc graben that partitioned the cordilleran arc into northern and southern segments like the sunda strait an extensional basin in indonesia sunda strait is a transition zone between java where the oceanic plate is subducting at right angles to the magmatic arc and sumatra where the oceanic plate is subducting at an oblique angle like the right lateral central fault in sumatra right lateral faults in the sierra nevada accommodated oblique subduction and facilitated extension where the subducting angle changed the late cretaceous structures that accommodated extension in the salinian terrane are not visible because of subsequent cenozoic tectonism which has slivered the terrane from a compact block into pieces that now extend from the transverse ranges to coastal locations north of san francisco

effective elimination of subharmonic ghost events from vibroseis data abstract harmonic or subharmonic noise is often present in vibroseis data as reverberation like laterally coherent bands occurring parallel to and before or after the main events such periodic noise is typically generated during the standard correlation process when the actual source signal travelling through the subsurface is for whatever reason different from the desired source signal i e the pilot sweep controlling the baseplate and used for correlation a typical cause can be that harmonic or subharmonic frequency partials are generated in addition to the vibroseis sweep s desired fundamental frequencies these harmonics produce strong ghost events during correlation of the geophone trace with the pilot sweep originating from additional correlations between the fundamental and harmonic frequencies especially subharmonic ghosts will overlap with good fundamental signals since for typically used up sweeps they are folded to later traveltimes where the signal noise ratio is already lower thus aggravating or preventing a reliable interpretation of possible later reflections here a method is introduced to remove these unwanted noise trains with only negligible impact on the fundamental signal by transforming the seismogram traces into a so called sub harmonic domain in this domain the respective harmonic noise portions are focused and separated from the fundamental signals enabling easier detection and appropriate suppression after back transformation to the x t domain the records are free from the corresponding harmonic contamination and can then be processed as usual the method operates in a data driven fashion i e the traces are not uniformly processed but are processed depending upon their actual sub harmonic content the decontamination procedure can be applied universally i e to uncorrelated correlated and or vertically unstacked stacked data either in a manual semiautomated or fully automated manner the method works perfectly for synthetic vibroseis traces with or without harmonic subharmonic portions the application to real crustal scale vibroseis records that were acquired in 2006 in the dead sea region israel and that were severely contaminated by subharmonic ground roll ghosts covering reflectivity from the basement to the moho shows the robustness and success of the presented method

sedimentology and reservoir potential of matilija sandstone an eocene sand rich deep sea fan and shallow marine complex california the matilija sandstone of the santa ynez mountains of southern california records a major regressive event in the eocene santa ynez basin in which turbidites were deposited and subsequently covered by shallow marine deposits despite thick favorable source beds and generally good initial reservoir characteristics the matilija sandstone is not a productive unit in the basin low reservoir permeability generally less than 1 md and porosity 1 to 10 are interpreted to be the result of early compaction cementation and diagenesis the lower part of the matilija sandstone is a 1 300 ft 400 m thick sand rich deep sea fan complex which overlies basin plain turbidite deposits of the juncal formation the matilija sandstone consists of anastomosing outer fan lobes lobe fringe and interlobe deposits overlain by channelized middle and inner fan deposits the upper matilija sandstone consists of shallow marine cross bedded sandstone red beds and carbonate evaporite lignite sequences which overlie the turbidites deposition of the matilija sandstone was terminated by a rapid basin deepening event which culminated in deposition of basin plain slope and turbidite deposits of the cozy dell formation the lower matilija sandstone deep sea fan deposits have a high sandstone to shale ratio 4 1 to 10 1 and consist of submature arkoses folk 1980 the depositional lobes consist of facies b c and d turbidites and a channelized sequence of facies a and b turbidites the average sandstone is medium grained moderately sorted subangular to subrounded and massive containing 40 quartz 35 feldspar about equal amounts of plagioclase and potassium feldspars 10 lithic fragments mostly volcanic and metamorphic types and minor amounts of detrital clay illite mica chert and heavy minerals early compaction carbonate cementation and authigenic pore lining and pore filling chlorite albite potassium feldspar quartz and localized laumontite have reduced the initial porosit and permeability the only remaining porosity is secondary resulting from the dissolution of detrital feldspars and minor fracture porosity

magnetostratigraphic tests of sequence stratigraphic correlations from the southern california paleogene abstract several sequence stratigraphic analyses have been conducted on the eocene oligocene rocks of the san diego region and of the western transverse ranges in southern california recently developed magnetic stratigraphy and 40ar 39ar dates as well as previously neglected biostratigraphic data provide a high resolution chronostratigraphic framework to test correlation of these sequence stratigraphic interpretations to the global onlap offlap chart haq et al 1987 1988 hardenbol et al 1998 in the san diego area previous stratigraphic analyses miscorrelated nearly all sequences and sequence boundaries to the global time scale some correlations are off by as much as ten million years in the western transverse ranges three contradictory sequence stratigraphic analyses are evaluated the revised dates of the sequence boundaries show poor correlation with the onlap offlap chart of haq et al 1987 1988 or hardenbol et al 1998 and some units are misdated by as much as six million years in both regions few sequences were correctly predicted by the onlap offlap chart suggesting that tectonic controls were more important than eustatic controls in this tectonically active region the errors in so many sequence stratigraphic correlations when tested by high resolution chronostratigraphic data should caution geologists about using sequence stratigraphic age assignments uncritically sequence stratigraphic correlations are only as good as the chronostratigraphy on which they are based

tensile and compressive growth structures relationships between sedimentation deformation and granite intrusion in the archaean coppin gap greenstone belt eastern pilbara western australia abstract the coppin gap greenstone belt is composed of an east west striking asymmetric synclinorium between the muccan and mount edgar batholiths along the contact with the muccan batholith the vertical to overturned northern limb is intensely sheared folded and tectonically reduced the southern limb is relatively undisturbed and irregularly intruded by the mount edgar batholith progressive unconformities and sedimentary infilling of growth structures in the gorge creek group record the deformation history the unconformities constrain which of the now steeply dipping shear zones have to be rotated with their cover sequence before they can be interpreted kinematically four mid archaean clastic systems have been recognized the first and third are transgressive littoral quartzose and feldspathic arenites the second and fourth comprise conglomeratic and ferruginous alluvial fan related deposystems east verging thrust tectonics affected the belt after unconformable deposition of the first deposystem on the early archaean warrawoona group in the western extremity of the belt primary tectonic relief was filled with the second deposystem thrust folds were overprinted at right angles by folding sub parallel to the developing belt after truncation the belt was covered by the third and fourth systems in the eastern part of the belt relief created by belt parallel folds affecting the first deposystem was filled by conglomerate overlain by basalts a sequence also assigned to the whim creek group we relate systematic depocentre shifts as observed in the bamboo creek syncline to differential doming of the adjacent batholiths buried thrusts and shear zones may have caused km scale repetition of the underlying sequences the observed relationships place uncertainty on the validity of the existing stratigraphic correlations which now need geochronologic work to resolve part of the thrusting appears to have taken place by inversion on pre existing detachment shear zones in the underlying warrawoona basalts and ultramafic rocks the shear zones descend from a high stratigraphic level in the east towards deeper levels in the west the first motion has been west block down listric normal growth faulting in places accompanied by synsedimentary explosive intrusion of black chert veins mega brecciation and back tilt of fault blocks geometrically these early structures cannot be related to the geometry of batholith doming and are interpreted as the result of either regional tensile stress local caldera collapse or both mechanisms acting synchronously the mid archaean influx of siliciclastic sediments in the coppin gap belt coincides with this change from a tensile to a complex compressive deformation the latter caused by regional thrusting and interbatholith folding late deformation of the belt includes further steepening of the strata and eastward oblique dextral and 060 sinistral strike slip

repeatability study of seismic source signatures this study discusses the repeatability of source signature using the instantaneous phase as derived from the complex trace attributes the study is part of a very large 2 d seismic survey using sources of vibroseis and surface dynamite the field procedure consisted of recording the production record retaining the positions then repeat recording the same shake or shot the instantaneous phase was found to be the best measure for the difference between the first and the repeat records in addition to the instantaneous phase other analyses were used to evaluate changes in source signature results were tabulated for statistical comparisons and graded for quality excluding erroneous cases the remainder of poor repeatabilities were studied the analyses of near offset data seem to indicate that nonrepeatability of source signature relates mostly to changes in absorption and cohesion induced by elastic saturation at the near surface in general by time shifting and phase rotating the repeat record the difference in instantaneous phase tends to diminish the new idea of using instantaneous phase difference plots to evaluate repeatability offers improved evaluation of source signatures and can also be used to detect time lapsed changes in reservoir monitoring by evaluating repeatability and avoiding elastic saturation near the surface source signatures can be made more consistent thus increasing the resolution of stacked data for 2 d 3 d and 4 d seismic surveys

quantifying the oldest tidal record the 3 2 ga moodies group barberton greenstone belt south africa the 3 2 ga moodies group in the barberton greenstone belt south africa contains the oldest preserved record of tides the tidal record is preserved in a tidal sand wave deposit in the lower moodies group as bundles of sandstone foresets separated by mudstone drapes detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal fortnightly and monthly tidal periodicities thick thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood tidal currents respectively cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap spring neap tidal cyclicity alternating thicker and thinner neap spring neap cycles are comparable to anomalistic perigean apogean tidal signatures this quantitative record of tides in the middle archean moodies group represents by 2 2 b y the oldest such documentation tidal cyclicity recognized in the moodies sand wave deposit is comparable to that recorded in modern tidal settings and identified in the carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

the construction of a simple portable electromagnetic vibrator from commercially available components since its introduction in the late 1950s hydraulic vibrators have become the dominant source for land seismic surveys the hydraulic vibrators typically used for commer cial land seismic acquisition however are large costly to operate and expensive to purchase this inhibits their use for small scale and short duration surveys as well as vibroseis research in this paper we describe in detail the construction of a portable vibrator from commercially available components for a cost of less than us2 000 data shows that the vibrator is able to successfully transmit sweeps from 15 to 180 hz with different spectral contents the vibrator produces a stronger signal than a sledge hammer and we estimate its output to be around 1 kn the frequency content of the data was concentrated at lower frequencies 100 hz and the ground roll was far more energetic than that produced using a sledgehammer

early archean 3 2 ga fe oxide rich hydrothermal discharge vents in the barberton greenstone belt south africa ironstone pods fe 2 o 3 72 5 97 2 wt interpreted to have formed by early archean sea floor related hydrothermal activity are described from the ca 3 5 3 1 ga barberton greenstone belt south africa most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy they rest on silicified ultramafic rocks and are overlain by ferruginous shales silicified sand stones other coarse clastics and barite horizons the ironstone pods grade along strike into laminated iron oxide facies banded iron formation that is inferred to represent periodic discharge of iron oxide and silica rich flocculates from the pods the massive texture of the iron stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor relict hydrothermal discharge chimney structures have been recognized honeycomb like cavities and possible fluid flow channel textures attest to primary porosity the iron stone pods are dominated by massive locally coarse grained specular hematite and goethite with lesser amounts of quartz and an x ray amorphous fe ai bearing silicate the pods are distinct from typical massive sulfide gossans in total trace element concentrations including precious and base metals indicating a different origin in contrast the ironstone pods have similarities in major oxide and trace metal concentrations to iron oxide deposits presently forming in part from low temperature hydrothermal fluids on the sea floor for example red seamount iron oxide deposit fluid inclusion studies on quartz show dominant type i primary fluid inclusions with salinities of 4 7 to 15 8 wt nacl equiv and homogenization temperatures t h of 90 to 150 c no evidence for boiling is seen type i fluid inclusions are most likely dominated by nacl cacl 2 h 2 o although their measured eutectic temperatures are consistent with the presence of fecl 2 these inclusions represent the end member hydrothermal fluid more saline 24 29 6 wt cacl 2 equiv lower temperature t h 33 109 c type ii and ila fluid inclusions represent pulses of a hydrothermal fluid of nacl cacl 2 fecl 2 or mgcl 2 h 2 o or possibly fecl 3 bearing composition mixing with ambient sea water in the case of type ii inclusions the various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine the lowest salinity measurement for a type i inclusion of 3 1 wt nacl equiv provides a constraint on the salinity of unenriched by evaporation sea water the t h data give a minimum trapping pressure of 7 bars which equates to a minimum sea water depth of 60 m the lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods quartz 18 o analyses average 17 3 0 6 1 n 5 calculated 18 o h 2 o for two samples are 1 4 and 1 2 respectively these estimates are within error of the postulated barberton early archean sea water value of 0 quartz hematite goethite mineral pairs yield temperatures inconsistent with fluid inclusion t h data suggesting isotopic disequilibrium

ground viscosity and stiffness measurements for near surface seismic velocity in land surveys the weathering layer can often distort the seismic signal due to it passing through rapid velocity and density changes dispersion scattering and inelastic absorption in a simple spring dashpot model for the earth response an equivalent medium groups these complex phenomena into two parameters only these are called ground viscosity and ground stiffness the most recent controllers for vibrators can estimate both parameters to validate these measurements saudi aramco conducted an experiment measuring ground viscosity and stiffness from two different vibrator control systems over an area of varying terrain conditions including unconsolidated sand and limestone outcrop the two systems measured different values but detected similar trends that correlated well with weathering conditions and surface geology e g lower viscosity values on the outcrop than on the sand the ratio of ground viscosity to ground stiffness can approximate the shallow s wave velocity which we converted into p wave velocity through calibration with sparse uphole data static corrections incorporating this velocity information somewhat improved the focusing of seismic time sections this new approach does not require additional acquisition efforts and can model shallow complex formations in arid areas where classical static methods often fail

seismic vibrator control and the downgoing p wave while the need for phase compensation is well established the best method to measure the seismic vibrator output is not phase control of the force exerted by a seismic vibrator upon the earth s surface ground force is shown to be useful in producing consistent downhole p wave signatures experimental results are presented which compare the downhole correlation wavelets produced by phase locking to ground force reaction mass acceleration and baseplate acceleration as changes in vibrator type sweep bandwidth drive level and coupling medium are made the empirical results support earlier theoretical work which predicts with suitable assumptions that ground force and far field particle displacement are in phase except for a time delay

enhanced servovalve technology for seismic vibrators the pelton dr tm servovalve enhancement causes the natural output of a vibrator to resemble the desired output more closely this simplifies the control problem and reduces harmonic distortion the traditional type of servovalve used on seismic vibrators is a flow control servovalve flow is proportional to a vibrator s baseplate velocity with respect to its reaction mass the new servovalve control parameter is pressure rather than flow the differential pressure applied to a vibrator s actuator piston multiplied by the area of the piston equals the force applied to the vibrator s baseplate structure this may be defined as actuator force there is a simpler and more linear relationship between actuator force and ground force than between actuator velocity and ground force thus it is better for the servovalve to control pressure into the actuator rather than flow a flow control servovalve can be made to control pressure by sensing the differential pressure across a vibrator s actuator piston and applying it as a negative feedback around the servovalve main stage this has been carried out and tested the result is more accurate vibrator control and reduced harmonic distortion

paleomagnetic definition of crustal fragmentation and quaternary block rotations in the east ventura basin and san fernando valley southern california 1 paleomagnetic studies of the pliocene quaternary saugus formation in the eastern part of the western transverse ranges of california show that the crust is fragmented into small domains tens of kilometers in linear dimension identified by rotation of reverse fault blocks in an area approximately 35 25 km in the san fernando valley and east ventura basin we identified four distinct domains two domains southwest of and adjacent to the san gabriel fault are rotated clockwise 1 the magic mountain domain r 30 5 and 2 the merrick syncline domain r 34 6 the magic mountain domain has rotated since 1 ma both rotated sections occur in hanging walls of active reverse faults the santa susana and san fernando faults respectively structural data suggest that the fault tip of the santa susana fault is the rotation pivot of the magic mountain domain two additional blocks are unrotated 1 the van norman lake domain directly south of the santa susana fault and 2 the soledad canyon domain immediately across the san gabriel fault from the magic mountain domain suggesting that the san gabriel fault might be a domain boundary our results suggest that part of the clockwise rotation of some miocene and older rocks in this area might have occurred in the quaternary the plio pleistocene fragmentation and clockwise rotations continue at present based on geodetic data and represent crustal response to diffuse oblique dextral shearing within the san andreas fault system

growth fault structure and stratigraphic architecture of the buck ridge volcano sedimentary complex upper hooggenoeg formation barberton greenstone belt south africa abstract the 3 45 3 42 ga buck ridge volcano sedimentary complex br vsc forms the uppermost part of the hooggenoeg formation in the early archaean onverwacht group of the barberton greenstone belt south africa the complex consists mainly of massive and pillowed basalts and felsic quartz plagioclase porphyritic rocks which are capped by pervasively silicified sedimentary rocks deposition of the felsic extrusive and volcani clastic sedimentary rocks occurred in shallow water to emersion during an extensional tectonic regime the extensional regime is expressed by a number of listric normal faults that transect the entire 2 km thick complex the syndepositional i e growth fault character of the faults is indicated by variations in thickness and facies of rock units across the faults and by systematic rotation of the fault blocks broadly contemporaneous with the faulting felsic rocks intruded the complex into the highest levels the geometry of these intrusive bodies was controlled by the major growth faults extension also created space for the intrusion of approximately bedding perpendicular felsic and mafic dykes and black chert veins the new field observations and u pb zircon shrimp data have pinned the timing of events during and after deposition of the br vsc the growth fault model accounts well for previously unexplained structural observations that include both extensional and compressional features recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the barberton greenstone belt and the greenstone belts of the east pilbara in australia

the use of pseudorandom sweeps for vibroseis surveys pseudorandom vibroseis sweeps have long been suggested as an alternative to standard linear sweeps due to their potential for having superior orthogonality a lower likelihood for infrastructure damage and increased low frequency content in the past they were also attractive as they have a better autocorrelation shape although that is less important today their use has been limited but the increasing popularity of simultaneous acquisition techniques has rekindled interest as they offer the ability to reduce interference noise a wide variety of methods for generating pseudorandom sweeps have been developed over the last 45 years this paper gives an overview of the motivations for their use before classifying and describing the different sweep types finally the sweeps will be compared in terms of their major attributes including their suitability for simultaneous surveys

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply comment lowe and byerly s 2003 1 description of iron oxide depositing spring terraces in the barberton greenstone belt and how they relate to the iron stone pods we studied is misleading and inaccurate here we demonstrate that the archean iron deposits we studied have no relationship to the iron

radiation from flexural vibrations of the baseplate and their effect on the accuracy of traveltime measurements we analyse the problem of radiation of seismic waves by a vibroseis source when the baseplate is subject to flexure a theoretical model is proposed to account for baseplate flexure generalizing the well known model of the vibroseis source of sallas and weber which was developed for a rigid plate using the model proposed we analyse the effect of flexure on the properties of seismic waves we show that the flexure does not contribute to the far field and mainly affects the readings of the reference accelerometer that is used to measure the force applied to the ground these readings generally become dependent on the location of the sensor on the plate for muddy and sandy soils the effect of flexure on baseplate acceleration measurements is nonetheless pronounced at the high end of the vibroseis frequency band only 100 hz and is negligible at all frequencies for stiffer soils the corresponding phase lags introduced by the flexural vibrations at high frequencies lead to errors in the traveltime measurements through the cross correlation function of up to 0 6 ms for muddy soils and less for denser soils we show the existence of an optimal position of the reference sensor on the baseplate and also propose a general method of eliminating the phase lag due to the baseplate flexure in acceleration measurements

flat pebble conglomerate its multiple origins and relationship to metre scale depositional cycles carbonate flat pebble conglomerate is an important component of precambrian to lower palaeozoic strata but its origins remain enigmatic the upper cambrian to lower ordovician strata of the snowy range formation in northern wyoming and southern montana contain abundant flat pebble conglomerate beds in shallow water cyclic and non cyclic strata several origins of flat pebble conglomerate are inferred for these strata in one case all stages of development of flat pebble conglomerate are captured within stormdominated shoreface deposits of hummocky cross stratified hcs fine carbonate grainstone a variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat pebble conglomerate these features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits failure was induced by either storm or seismic waves and many beds failed along discrete slide scar surfaces centimetre scale laminae within thick amalgamated hcs beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds in some cases buckled masses accelerated downslope until they exceeded their internal friction completely disarticulated into clasts and transformed into a mass flow of individual cm to dm scale clasts this transition was accompanied by the addition of sand sized echinoderm rich debris from local sources which slightly lowered friction by reducing clast clast interactions the resulting dominantly horizontal clast orientations suggest transport by dense viscous flow dominated by laminar shear these flows generally came to rest on the lower shoreface although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds the clasts in these beds show no evidence of extensive reworking i e not well rounded or condensation i e no rinds or coatings a second type of flat pebble conglomerate bed occurs at the top of upward coarsening metre scale cycles the flat pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or in some cases may represent transgressive lags the clasts are well rounded display borings and have iron rich coatings the matrix to these beds locally includes glauconite these beds were considerably reworked and represent condensed deposits thrombolites occur above the flat pebble beds and record microbial growth before initial transgression at the cycle boundaries a third

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

the present state of seismic data acquisition one view seismic data acquisition in the mid 1980s is briefly reviewed in terms of hardware the trend has been toward an increased number of data channels in both land and marine applications this has led to the development of digital telemetry systems positioning systems particularly for marine work have made use of artificial satellites the perceived need for s wave information has led to development of s wave sources such as the horizontal vibrator s waves in a few cases have been used to validate hydrocarbon indicators on seismic records there has been a distinct trend toward three dimensional 3 d seismic recording both on land and at sea and for both exploration and production applications

source measurement effect on high fidelity vibratory seismic separation high fidelity vibratory seismic hfvs acquisition and separation can play an important role in today s land acquisition schemes the method in which multiple vibrators are swept simultaneously using sweeps with known phase encoding and then the data are inverted and separated into individual records can improve productivity in the field and at the same time improve signal characteristics in the data it relies on the measured weighted sum of accelerations base plate and reaction mass to invert the acquired data and separate the individual vibrator responses separation can be sub optimal if the measured motions vary from the true source input into the ground differences in true source andmeasured source can arise due to poor coupling between vibrators and ground soil compaction or other factors using both a synthetic model and real data we show that if the true source changes between sweeps but is not measured vibrator responses can leak into adjacent vibrator responses upon separation in a recent survey with hfvs acquisition we observed a 25 30 db separation between adjacent vibrators which could be improved with greater reliability of the source measurement the vibrator leakage can reduce the data quality considerably we discuss the results of this survey and show that separation is affected by source measurement error further we conclude that it is necessary either 1 to use source measurements that can capture the variability of the true source between sweeps or 2 to compensate for the source measurement variations in processing or in acquisition

analyses of vibrator and geophone behavior on hard and soft ground the vibroseis method has for half a century achieved great success in land seismic exploration however some practical issues still arise that have remained theoretically unexplained for example on soft ground the vibrator produces subharmonics and ultra subharmonics in addition to main harmonics whereas on hard ground the vibrator generates harmonics only geophones on soft ground also behave abnormally while geophones on hard ground behave normally this paper analyzes these phenomena and demonstrates that the softness of the ground s top layer is responsible for subharmonics and ultra subharmonics this soft ground layer causes the geophone abnormality as well unfortunately quantification for this behavior has not been achieved

seismic interferometry experiment in a shallow cased borehole using a seismic vibrator source we present the results of a seismic interferometry experiment in a shallow cased borehole the experiment is an initial study for subsequent borehole seismic surveys in an instrumented well site where we plan to test other surface borehole seismic techniques the purpose of this application is to improve the knowledge of the reflectivity sequence and to verify the potential of the seismic interferometry approach to retrieve high frequency signals in the single well geometry overcoming the loss and attenuation effects introduced by the overburden we used a walkaway vertical seismic profile vsp geometry with a seismic vibrator to generate polarized vertical and horizontal components along a surface seismic line and an array of 3c geophones cemented outside the casing the recorded traces are processed to obtain virtual sources in the borehole and to simulate single well gathers with a variable sourcereceiver offset in the vertical array we compare the results obtained by processing the field data with synthetic signals calculated by numerical simulation and analyse the signal bandwidth and amplitude versus offset to evaluate near field effects in the virtual signals the application provides direct and reflected signals with improved bandwidth after vibrator signal deconvolution clear reflections are detected in the virtual seismic sections in agreement with the geology and other surface and borehole seismic data recorded with conventional seismic exploration techniques

on the generation of low frequencies with modern seismic vibrators abstractacquiring low frequency seismic data using vibroseis techniques has gained attention in recent years successful application of low frequency vibroseis acquisition requires evaluating each element of the data acquisition system and ensuring that each part of the system contributes to the success of the method we focus on generating low frequencies with vibroseis sources rather than recording and preserving them to generate the low frequency signals using seismic vibrators it is better to know 1 the hydraulic and mechanical system of the vibrator 2 the design of a low frequency sweep that can deliver the vibrator ground force maximally and 3 the vibrator control system that can minimize the harmonic distortion of the vibrator ground force output at low frequencies field tests show that hydraulic pump flow is not a limiting factor for the generation of the low frequencies for most modern seismic vibrators in addition with harmonic distortion reduction control the vibrator performance

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

origin and mechanisms of k si metasomatism of ca 3 4 3 3ga volcaniclastic deposits and implications for archean seawater evolution examples from cherts of kittys gap pilbara craton australia and msauli barberton greenstone belt south africa abstract the effects of k si metasomatism during the formation of early archean replacement cherts have been quantified in this study by the investigation of two well known stratigraphic sections the msauli chert mc barberton greenstone belt south africa and the kittys gap chert kgc pilbara craton western australia the kgcs have a dacitic precursor similar to duffer formation dacites pilbara craton while the mcs are derived from al depleted komatiites similar to those from the weltevreden formation barberton greenstone belt mass balance calculations reveal that the volcaniclastic deposits had initial porosities of up to 85 vol for the kgc and of 65 vol for the mc secondary porosities 27 vol mc 8 vol kgc produced during k metasomatism are proportional to the dissolution of fe ca mg rich glass and precursor minerals komatiites have a higher chemical exchange potential than dacites each gram releasing 1 2 mmol fe2 2 8 mmol mg2 1 4 mmol ca2 and 1 1 mmol na to seawater together with 4 4 mmol o2 k metasomatism of 1 g of komatiite further implies an uptake of 0 67 mmol of k and 2 7 mmol of h the highest silica uptake is achieved for the kgc 82 mmol g of precursor this silica enrichment most likely operated in the water column and at the sediment water interface by sorption mechanisms on the surface of detrital particles and particulate organic matter as a result of seawater silica saturation acidic conditions ph 5 5 6 5 and hot temperatures 70 c favored the formation of k rich phyllosilicates by interaction with seawater during the early diagenetic alteration of the volcaniclastic particles the widespread occurrence of k si metasomatism in volcanic and sedimentary rocks can be regarded as a general alteration process of the early archean seafloor with a major influence on seawater composition the highly k selective metasomatism confirms previous studies suggesting that the archean ocean was acidic and probably in equilibrium with a co2 rich atmosphere

photosynthetic microbial mats in the 3 416 myr old ocean recent re evaluations of the geological record of the earliest life on earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2 3 similarly many early archaean more than 3 400 myr old cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2 4 5 here we present the results of a field petrographic and geochemical study testing these hypotheses for the 3 416 myr old buck reef chert south africa from sedimentary structures and distributions of sand and mud we infer that deposition occurred in normal open shallow to deep marine environments the siderite enrichment that we observe in deep water sediments is consistent with a stratified early ocean6 7 we show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments we find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments instead we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8 9 3 416 myr ago

tectonic rotation of the santa ynez range california recorded in the sespe formation summary terrestrial claystone and siltstone in the sespe formation oligocene in the santa ynez range near santa barbara california record a palaeomagnetic field that predates the regional folding and satisfies a reversal test the mean declination of 80 1 closely matches the declination in miocene volcanic and sedimentary rocks studied by others in the western transverse block of which the santa ynez range is a part and the mean inclination of 38 9 is consistent with a hypothesis that the block was once about 10 south of its present latitude the combined palaeomagnetic data indicate that clockwise rotation of the block is entirely post oligocene the rotation might not haye commenced until the middle miocene or about 15 myr after northward motion of the block began

modelling and modal analysis of seismic vibrator baseplate the vibroseis method must be extended to its limits as the search for oil and gas continues on land to successfully improve vibroseis data quality it is crucial to evaluate each element in the vibroseis data acquisition system and ensure that the contribution from each element is successful vibroseis systems depend greatly upon the ability of vibrators to generate synchronous repeatable ground force sweeps over a broad frequency range this requires that the reaction mass and the baseplate of the vibrator move as rigid bodies however rigid body motion is not completely true for high frequency vibrations especially for the vibrator baseplate in order to accurately understand the motion of the vibrator baseplate a finite element analysis model of the vibrator baseplate and the coupled ground has been developed this model is useful for simulating the vibrator baseplate dynamics evaluating the impact of the baseplate on the coupled ground and vibrator baseplate design model data demonstrate that the vibrator baseplate and its stilt structure are subject to six significant resonant frequencies in the range of 10 80 hz due to the low rigidity of the baseplate the baseplate stilt structure experiences severe rocking motions at lower frequencies and the baseplate pad experiences severe flexing motions at higher frequencies flexing motions cause partial decoupling which gives rise to increased levels of harmonic distortion and less useable signal energy in general the baseplate pad suffers more bending and flexing motions at high frequencies than low frequencies leading less efficiency in transmitting the useable energy into the ground

paris basin vsps case history establishing combinations of fine layer or lithologic anisotropy and crack anisotropy from modelling shear wavefields near point singularities summary this paper examines shear wave splitting in multi offset vsps at a borehole site in the paris basin and demonstrates that the sedimentary structure must have at least orthorhombic anisotropic symmetry it is suggested that the most likely cause for such an orthorhombic symmetry is the combination of fine layer or lithologic anisotropy and crack anisotropy transition zones close to shear wave singularities where there are rapid changes of shear wave polarizations and delays can account for the anomalous observed waveforms observations of large variations in shear wave polarizations in propagation directions close to point singularities are probably the first positive identification of the effects of singularities in sedimentary basins

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

geophysical benefits from an improved seismic vibrator the seismic vibrator has become a very important source for land data acquisition and there have been dramatic improvements in recent times in the application of the vibroseis technique these improvements have led to much increased productivity and in many cases much denser source sampling at the same time the vibrator itself has seen little improvement over the last couple of decades there are needs in a few areas where an improvement in the vibrator itself can bring benefits to the quality of the seismic data acquired this paper describes progress in four such areas low frequency performance high frequency performance an improved estimate of the vibrator groundforce and source signature consistency over variable ground conditions each of these vibrator characteristics will be discussed in turn meanwhile two field test results in which the performance of two different vibrators in these four areas are compared

stratigraphic continuity and early deformation of the central part of the coppin gap greenstone belt pilbara western australia abstract this article focuses on the discrepancy between the often advocated stratigraphic continuity of greenstone belts in the pilbara craton australia on the one hand and the observations of syndepositional deformation and other tectonic disturbance on the other hand the coppin gap greenstone belt of the east pilbara consists predominantly of mafic intermediate and felsic rocks with minor amounts of silicified sedimentary rocks of the early archaean warrawoona group several well defined zones subparallel to the bedding show intense brittle to ductile deformation testifying to differential movements of the tectono stratigraphic slices they delimit these deformation zones were at least partly related to west block down normal faults and accommodated upper block to the west transport however new u pb zircon dates geochemical analyses and detailed field observations show that the overall stratigraphic succession in the coppin gap greenstone belt has not been fundamentally disturbed the normal faults occur in several irregularly superposed arrays of km spaced faults part of which were active during deposition normal faults also occur throughout the entire belt combined the early deformation structures indicate east west extension over a minimum period of 20 ma from 3469 ma onwards likely the extension took place over an even longer period of time during much of the time of deposition of the warrawoona group from 3490 ma onwards syndepositional extension of the warrawoona group was geometrically unrelated to the doming of granitoid batholiths surficial i e uppermost crustal gravitational collapse during or shortly after deposition may have caused the apparent contradictory co existence of deformation zones and stratigraphic continuity although the deformation has not caused major disruptions of the stratigraphic succession the precision of the u pb shrimp ages is insufficient to rule out excisions or duplications of stratigraphy on the order of a few million years

on the accuracy of the ground force estimated in vibroseis acquisition for a linear elastic earth the time derivative of the ground force is considered proportional to the far field wavelet under the assumption that the baseplate is stiff and the bending forces of the baseplate are negligible the ground force is also approximated by the sum of the accelerations of the baseplate and the reaction mass weighted by the respective masses combining these two assumptions the time derivative of the weighted sum is considered proportional to the far field wavelet this result often referred to as the far field wavelet assumption although convenient and most often employed is not always valid we explore its validity using the spectral harmonic ratios of recorded data which are used extensively in data filtering and analysis of vibratory data we show that the far field wavelet assumption fails particularly for harmonic components of even order more compact soil after repeated shots further invalidates this assumption non linear modelling of the ground under the vibrator point may provide a direction towards solving this discrepancy finally we describe a method for the estimation of the harmonic spectral ratios

stratigraphic and sedimentological evidence bearing on structural repetition in early archean rocks of the barberton greenstone belt south africa abstract the 3 5 3 3 ga old swaziland supergroup in the barberton greenstone belt south africa includes a lower predominantly volcanic unit the onverwacht group and an upper largely sedimentary section the fig tree and moodies groups although viljoen and viljoen concluded that the onverwacht group exceeds 15 km in stratigraphic thicknesses more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting our mapping indicates that the upper half of the group includes at least 8 km of unrepeated stratigraphic section inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the onverwacht group on the west limb of the onverwacht anticline shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform highly complex zone of deformation up to 2 km wide the displacement across the zone is unknown but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the group has not been significantly altered structures previously mapped as major fold hinges within the upper part of the onverwacht group can be shown to have formed by local deformation and reflect neither large scale folding nor structural repetition of the rocks several kilometers higher in the section rocks of the uppermost onverwacht and fig tree groups are stacked and repeated in a series of thrust nappes individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks they were apparently emplaced by faulting or gravity sliding at or near the earth s surface their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying much weaker serpentinized ultramafic komatiites locally the sheets were deformed to form fold nappes the lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact the thrust faults are the loci of small hypabyssal igneous bodies that appear to be co magmatic with a suite of upper fig tree volcanic rocks this style of faulting has not affected rocks of the overlying moodies group although estimates of the timing of deformation are as yet imprecise these features suggest that it occurred during a late fig tree phase of tectonism and volcanism

seismites an attempt at critical analysis and classification abstract the main types of seismites are reviewed following the proposed classification and are illustrated by our own case studies the large variety of seismites depends on the sedimentologic hydrodynamic and diagenetic characters of the deposits subject to seismic shocks on a given site generally different types of seismites cane be related to the same seismic event these peculiar but widespread sedimentary structures are not always univocal and may often be related to different causes therefore the structures discussed must necessarily be placed in a detailed geological context

structures that accommodated differential vertical axis rotation of the western transverse ranges california 1 new paleomagnetic and structural data from the northwestern transverse ranges of california define the location geometry and kinematics of structures that facilitated differential vertical axis rotation paleomagnetic declinations in the southern coast ranges indicate negligible amounts of vertical axis rotation which contrasts sharply with data from rocks of the same age in the western transverse ranges that record approximately 90 of clockwise rotation since early miocene time this change in paleomagnetic declinations occurs across an east west trending zone of reverse faults and folds that includes the western big pine pine mountain fault the santa ynez fault and the structures in between the structures in this zone exhibit increased amounts of shortening to the west such that the zone closed in a fan like fashion resulting in rotation of the southern edge with respect to the northern edge the new data and interpretations refine previous models of vertical axis rotation in southern california by describing the deformation present at rotation boundaries that allowed distinct areas to rotate relative to the surrounding crust

a compaction correction for the paleomagnetism of the cretaceous pigeon point formation of california the paleomagnetism of the cretaceous pigeon point formation turbidites was reexamined to determine whether the 25 of southerly paleolatitudinal offset originally observed champion et al 1984 for these rocks was all or in part due to compaction shallowing of their paleomagnetic inclination the study consisted of two parts 1 a standard paleomagnetic study including detailed thermal and alternating field demagnetization was conducted on oriented cores collected at pigeon point approximately 50 km south of san francisco california the results of this study were combined with the alternating field demagnetized results for samples provided by d champion from the initial pigeon point paleomagnetic study the combined data set has a mean direction for pigeon point i 41 6 d 346 9 similar to that originally obtained by champion et al 1984 2 material from the pigeon point formation was disaggregated given a laboratory analog of a postdepositional remanence and compacted to pressures as high as 0 13 mpa which caused volume losses up to 53 the laboratory compacted samples were alternating field demagnetized and their magnetic inclination and anisotropy of anhysteretic remanence were both measured these data were used to derive correction curves following jackson et al 1991 which describe the specific relationship between remanence anisotropy and inclination shallowing for the pigeon point formation two correction curves were determined one assuming that the magnetic particle orientation distribution experienced a prolate deformation after remanence acquisition and one assuming an oblate deformation these two different corrections were necessary because the anisotropy of anhysteretic remanence indicates a composite fabric due to both a prelithification technically caused lineation and a burial compaction foliation the anisotropy of anhysteretic remanence measured for each paleomagnetic sample and the correction curves determined from the laboratory compaction experiments indicate that the inclination of the pigeon point formation has been shallowed by burial compaction the compaction corrected pigeon point mean directions assuming either a prolate i 53 1 d 347 2 or an oblate i 49 8 d 346 8 deformation suggest only 13 to 16 of southerly paleolatitudinal offset for the pigeon point formation in the cretaceous not the 25 originally observed champion et al 1984 the resulting paleolatitude for the pigeon point formation could indicate that salinia served as a link between the cratonic sierra nevada arc to the north and the peninsula ranges baja borderlands allochthon to the south alternatively our results suggesting a 10 compaction inclination shallowing for the pigeon point turbidites may indicate that many of the paleomagnetic studies placing the peninsula ranges baja borderlands 15 to the south of north america in the cretaceous and tertiary may have suffered from a similar effect and that the allochthon has been nearly in place since the cretaceous

how do hydraulic vibrators work a look inside the black box in order to have realistic expectations of what output is achievable from a seismic vibrator an understanding of the machine s limitations is essential this tutorial is intended to provide some basics on how hydraulic vibrators function and the constraints that arise from their design with these constraints in mind informed choices can be made to match machine specifications to a particular application or sweeps can be designed to compensate for performance limits

magnetic polarity stratigraphy and ages of siwalik group rocks of the potwar plateau pakistan abstract six magnetic polarity sections have been established over the potwar plateau region of pakistan including the major stratotypes of the siwalik group in all six sections the dominant feature of the magnetic polarity stratigraphy is a long normal polarity zone which is contained within the nagri formation this conspicuous normal polarity zone has been radiometrically dated at 9 5 0 6 m y which identifies it as magnetic chron 9 radiometric dates from the upper siwalik formation have also been used to identify the chron 2 3 boundary in two of the sections the magnetic polarity stratigraphy of three of the sections has been correlated securely with the accepted magnetic polarity time scale so that the ages of the local stratigraphy are indexed accordingly based exclusively on data from stratotype sections the chinji nagri and dhok pathan formations have nominal age ranges of 10 1 13 1 7 9 10 1 and 5 1 7 9 m y age fluctuations on the order of 105 years may be anticipated for these formational boundaries within radius of some 20 km of the designated stratotype mean sediment accumulation rates during the chinji nagri and dhok pathan interval range from 13 to 52 cm 103 yr essentially linear sediment accumulations are locally maintained over time intervals of several million years the chinji nagri lithofacies boundary marks a transition from slow to faster sediment accumulation over much of the potwar plateau indicating a fundamental sedimentary tectonic change at this time

acquisition and processing of simultaneous vibroseis data numerous methods have been developed in the past three decades to speed up vibroseis acquisition this paper gives an overview of the most promising ones and proposes a classification of them into three categories simultaneous shooting cascaded sweeps and slip sweep the principles upon which these methods are based the processing techniques developed to separate the simultaneously acquired data and their main features are also discussed finally some criteria for the selection of the most suitable methods for the acquisition and the separation stages are proposed

break through the limits of vibroseis data quality the vibroseis method has become the principal data acquisition method in land seismic exploration it seems that this method has been extended to its limits as the search for energy resources continues many practical issues arising from field operations have remained theoretically unexplained for example variations in wavelet arrival time inaccurate wavelet estimation and harmonics in the wavelet itself the focus of this paper is the proposal of a new model which is referred to as the vibrator coupled ground model to simulate the filtering effects of a complex coupling system consisting of the coupling between the baseplate and the ground as well as the coupling between the captured ground mass near the vibrator baseplate and the surrounding earth with this vibrator coupled ground model many of the practical issues mentioned above were reasonably addressed furthermore it was demonstrated from experimental tests that both the pilot sweep and the weighted sum groundforce when filtered by the vibrator coupled ground model are proportional to the far field particle velocity whereas the unfiltered signals are not the harmonics on the filtered weighted sum groundforce successfully maintain a proportional relationship with the harmonics seen in the far field signal

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

the amplitude and phase response of a seismic vibrator the amplitude and phase response of a simple model is compared with the performance of a real vibrator working in the field the field results show a characteristic phase response which confirms that the real drive force applied to the baseplate and its load impedance is faithfully represented by the acceleration of the reaction mass it follows that all the parameters necessary to calculate the load impedance and the true power dissipated in the earth can be measured at the output of the vibrator it also follows that the current method of baseplate phase compensation should be reconsidered

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs bodies of iron oxide to 200 m across in the 3 5 3 2 ga barberton greenstone belt south africa have been interpreted as deposits of archean seafloor hydrothermal vents and have provided what are arguably key observations about surface environments on early earth these bodies termed ironstone pods have yielded what are putatively the oldest known complex organic compounds and have been used to estimate archean surface temperature ocean depth and volume and seawater composition and to deduce relationships between hydrothermal activity and seafloor sedimentation however the pods are composed largely of goethite a thermally unstable hydrated iron oxide mineral show abundant still open primary porosity are undeformed in a terrane of highly deformed rocks and show bedding and cavity filling dripstone indicating formation with their present orientation they are here interpreted to have formed as spring and shallow subsurface deposits of young quaternary groundwater and or low temperature hydrothermal systems iron was derived by the dissolution of siderite in surrounding archean sedimentary rocks along pod margins goethite locally replaces archean chert and silicified ultramafic rock however coarse vein and cavity fill quartz was less susceptible to replacement and remains as disrupted masses and rare remnant veins in the ironstone this quartz is of archean age but contains no more information about the archean surface environment than other late hydrothermal veins throughout the barberton greenstone belt rocks the presence of a well preserved modern iron oxide spring terrace confirms that these are deposits of young subaerial springs and contain no record of archean life or environments they do however provide a unique view of a remarkable young iron depositing hydrologic system

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply reply we respond to de ronde et al as per their numbered comments 1 de ronde et al suggest that there are two populations of ironstone pods in the barberton greenstone belt those on farm mendon that are archean in age and composed primarily of hematite and all other ironstone pods which are young

early archaean hydrothermal fluids a study of inclusions from the 3 4 ga buck ridge chert barberton greenstone belt south africa abstract fluid inclusions have been studied from cavities and breccias in one of the oldest well preserved relatively low grade metamorphosed sedimentary sequences on earth the fluid inclusion bearing samples are from the uppermost part of the early archaean 3 4 ga buck ridge volcano sedimentary complex in the barberton greenstone belt south africa which was affected by hydrothermal alteration during and immediately after its deposition in banded silicified sandy sediments this alteration is characterised among other features by quartz cemented hydrothermal breccias layer bound stockworks of cm size quartz veins and quartz filled cavities below stacked pebbles quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two phase aqueous or mixed h 2 o co 2 fluid the aqueous inclusions are divided into three different groups based on their microthermometric characteristics type h a relatively high salinity type m a intermediate salinity and type l a relatively low salinity optical and microthermometric characteristics of the fourth type of inclusions the mixed h 2 o co 2 fluid type c indicate that it was homogeneous at the time of trapping with only some minor changes occurring after trapping type h a and type c fluids appear as immiscible fluids in their host crystal most probably resulting from unmixing of the hydrothermal fluid phase relations in the co 2 h 2 o system show that the unmixing must have occurred at a relatively low pressure of 100 bars this conclusion is in line with the geological context of the samples which indicates a shallow environment of deposition for the sediments almost contemporaneous hydrothermal activity and a very low metamorphic grade however it is contrary to the first impression given by the homogeneous character of type c inclusions which suggests much higher p t conditions at the time of trapping the aqueous component of type c fluids may have progressively mixed with high salinity aqueous fluids type h a resulting in a dilution trend from h a to m a to l a fluids the hydrothermal fluids from the buck ridge chert in the barberton greenstone belt are strikingly similar to those encountered in present day systems they show minor differences with older ca 3 8 ga fluids preserved in hydrothermally altered rocks from the isua greenstone belt in greenland

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

new possibilities in controlled source seismology with a magnetic levitation vibrator summary a new technology broad band seismic land source based on the principle of magnetic levitation is under development the new source is designed to overcome limitations experienced with controlled signal seismic sources at the present time and to create new possibilities in particular the new source extends the constant peak force frequency band down to 2 hz and is capable of at least several weeks continuous coherent transmission it will enable three dimensional long range 200 km controlled signal seismology with the potential for active earth tomography down into the mantle and systematic earthquake precursor measurement and should improve shallow and deep seismic profiling technology the optimum transmission frequencies depend on q transmission range and noise spectrum

magnetic stratigraphy and tectonic rotation of the middle eocene matilija sandstone and cozy dell shale ventura county california implications for sequence stratigraphic correlations abstract the matilija sandstone and cozy dell shale in the western transverse ranges of california record a long history of eocene regression from deep marine turbidites to non marine red shales and then transgression to deep marine shales in a rapidly subsiding tectonic basin in its type area along the ventura river north of ojai the matilija sandstone consists of over 800 m of deep marine to non marine sandstones and the cozy dell shale contains over 760 m of deep marine shales and turbidite sandstones paleomagnetic sampling of these rocks yielded a stable magnetic remanence which passed both a fold test and a reversal test some samples were reversed at nrm after dip correction based on calibration from planktonic microfossils the matilija sandstone spans the interval from chron c21r to early chron c20r 46 0 48 5 ma and the cozy dell from late chron c20r to c19r 46 0 42 0 ma the samples show a tectonic rotation of approximately 85 18 clockwise consistent with previous results reported for this tectonic block previous sequence stratigraphic correlations of these rocks are inconsistent with this chronostratigraphy none of the sequence boundaries predicted from these sections matches the global sequence chart