conceptual model for early diagenetic chert and dolomite amuri limestone group north eastern south island new zealand the late cretaceous to early eocene dominantly micritic amuri limestone group alg was deposited in an approximately nw trending trough in eastern marlborough new zealand the alg comprises the mead hill formation the teredo lower and middle limestone formations and the upper and lower marl formations chert and dolomite are concentrated in the mead hill formation which contains five of six recognized diagenetic zones zone i at the base of the alg consists almost entirely of chert zone ii consists solely of chert and dolomite zone iii comprises chert and limestone zone iv is composed of chert plus dolomite zone v is a chertified mudstone and the minor amounts of chert found in the middle limestone formation comprise zone vi with the exception of zones iv and v chert decreases stratigraphically upwards and away from the basin centre all the dolomites are composed of 1 mm diameter rhombohedra in discontinuous beds and lenses generally ca rich and non to slightly ferroan the dolomite contains approximately 500 900 ppm mn and 200 400 ppm sr 13c values average 1 2 pdb with 18o ratios of about 4 pdb mass balance calculations indicate that the mg2 for dolomitization was derived from sea water sr fe and mn concentrations are interpreted as indicating dolomite formation in the marine environment with no influence from meteoric waters the intimate association with pyrite implies dolomite formation in association with sulphate reduction in the upper sediment column 18o data show that the bulk of the dolomite formed at temperatures below 50 c all chert samples contain in excess of 90 wt sio2 about 1 wt al2o3 and 1 wt from losses on ignition generally all other major elements total less than 2 wt oxide 18o values range from 26 8 to 29 0 smow chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate bound components from the site of chertification and the effective dilution by sio2 of non carbonate bound insoluble residues 18o data indicate that chert formed in fluids of similar composition and temperature as the dolomite the abundance of disseminated pyrite in cherts implies an association with sulphate reduction silica for chertification is thought to have initially come from dissolution of siliceous organisms however there is insufficient biogenic silica available to form the volumes of chert observed it is suggested that the bulk of the silica came from sio2 rich pore waters generated by clay mineral reactions in the thick underlying mudstones the alg compacted down through these pore waters chert and dolomite nucleation are considered to have been penecontemporaneous dolomitization was initially probably the faster process continuing as long as sulphate reduction prevailed and there was an adequate supply of mg2 the nucleation of chert although initially slower probably due to a relatively lower initial sio2 supply continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix the amount of chertification decreased progressively as sio2 supplies diminished both stratigraphically upwards and away from the basin centre

triassic bedded cherts in central japan are not pelagic although the origin of bedded radiolarian cherts in orogenic belts has been widely debated1 15 their geochemistry is still poorly understood triassic bedded radiolarian cherts occur extensively in the japanese islands and are generally not associated with ophiolitic rocks but rather with clastic rocks we present here chemical data on the triassic bedded cherts of kamiaso in central japan the relationship of sio2 with al2o3 tio2 and zr shows that the bedded cherts have been formed by a simple mixing of biogenic silica derived from radiolarians and sponge spicules with lithogenic materials the ratios of several elements relative to tio2 strongly suggest that the shale partings and lithogenic portions of the cherts are consanguineous sedimentary structures as well as the geochemical evidence suggest that most kamiaso cherts are turbidity current deposits our geochemical and geological data compared with those of recent marine sediments from various environments demonstrate that the triassic cherts and associated shale partings are not deep pelagic in origin but formed in an offshore or marginal sea environment

observations on attenuation and shear wave velocity in fine grained marine sediments compressional p wave attenuation shear s wave attenuation and shear s wave velocity measurements compiled for fine grained unconsolidated sediments show trends that do not support empirical relationships hamilton j acoust soc am 68 1313 1340 1980 in acoustics and ocean bottom f a s e specialize conf madrid 1987 commonly used for geoacoustic modeling the exception is compressional wave attenuation data from 10 hz to 500 khz that essentially follow a frequency dependence of f1 2 standard deviations p wave attenuation with depth is difficult if not impossible to predict given the variability in the data examination of several attenuation depth profiles indicates that those of mitchell and focke j acoust soc am 67 1582 1589 1980 may be the best choice for predicting p wave attenuation with depth at the present time the assumption that s wave and p wave attenuations are proportional hamilton j acoust soc am 60 334 338 1976c in acoustics and ocean bottom

petrography and diagenesis of cherts from lake magadi kenya pleistocene cherts from alkaline lake magadi kenya are believed to have formed inorganically from sodium silicate precursors magadiite nasi 7 o 13 oh 3 3h 2 o and or kenyaite nasi 11 o sub 20 5 oh 4 3h 2 o they have been cited as modern analogs for cherts found in both ancient lacustrine deposits and precambrian iron formations but few criteria exist to distinguish magadi type chert from other cherts in the geological record we studied approximately twenty chert samples and one magadiite sample from lake magadi to develop such criteria and to shed light on the mechanism of the magadiite quartz conversion characteristics of the type magadi cherts are 1 a groundmass mosaic of fine quartz crystals that vary in orientation from random to rectilinear 2 finely disseminated inclusions of silicate clays zeolites and or carbonates predominantly calcite 3 large crystal molds probably after trona concentrated near sample margins and 4 inward directed shrinkage cracks and or more irregular internal voids filled with chalcedony silicate clays zeolites and or carbonates predominantly calcite shrinkage cracks which define surface reticulation patterns are frequently cited as evidence of a magadi type origin for ancient lacustrine cherts but they are not unique to alkaline lake cherts based on a reconnaissance of ancient cherts of various origins no single characteristic appears to be unique to magadi type chert with the possible exception of the rectilinear or grid work orientation of the quartz crystals this texture appears to be inherited from the precursor magadiite which displays a similar extinction pattern due to the presence of 10 20 mu m spherical aggregates of plate like crystals this pseudomorphing implies direct volume for volume replacement of magadiite by quartz void filling chalcedony and the high densities of the cherts however indicate that silica was added during and after the conversion from magadiite to chert petrographic observations suggest the magadiite quartz conversion takes place in multiple stages with early conversion in brines and later leaching by more dilute meteoric water a reconnaissance of 35 chert samples from 16 different formations ranging in age from pleistocene to archean yielded only two samples with convincing grid work fabrics both of which had already been interpreted as magadi type chert on the basis of other characteristics this suggests that the distinctive textures which characterize the type magadi cherts may be useful for recognizing other inorganic alkaline lake cherts in the stratigraphic record

quantifying the interaction of an ocean bottom seismometer with the seabed a theory is presented for the coupling between an ocean bottom seismometer obs the sediments upon which it rests and the surrounding water assuming that rotational and tilt effects are negligible or have been made negligible through instrument design the response of the obs to forced harmonic motion is considered in both horizontal and vertical directions under these conditions it is concluded that the measured ratio of velocities when the obs is on the seabed and when it is freely suspended in water for an identical force completely characterizes the obs seabed interaction this enables the velocity transfer functions to be directly calculated without recourse to a detailed model of sediment structure interaction an obs was designed and constructed with good coupling to seabed motion and reduction of rocking effects as principal design criteria including an onboard shaker to conduct in situ coupling experiments the amplitude and phase of coupling data collected on a clay seabed provide transfer functions due to horizontal and vertical seabed motion and horizontal water motion in addition a simple mass spring dashpot model of obs seabed interaction permits the analysis of amplitude only coupling data good vertical coupling can be achieved over a wide bandwidth by designing the sensor package to have a large hydrodynamic added mass in the vertical good coupling to horizontal seabed motion is more difficult to achieve but is possible within a limited bandwidth even on very soft seabeds finally an example of seismoacoustic noise is presented hydrophone signals are compared with horizontal geophone signals received from sources within the ocean and within the seabed and the differences are explained in terms of the coupling transfer functions

examples of strike slip tectonics on platform basin margins abstract examples from the southern alps north italy serve to illustrate the use of carbonate platform basin relations to recognize strike slip tectonics and to estimate amounts of displacements it is noted that two dimensional cross sections across strike slip faults cannot be balanced and three dimensional reconstructions would be necessary to restore original paleogeographic shapes of platform basin margins the main characteristics of strike slip tectonics are 1 a tectonic omission of fragments of the original carbonate platform basin system in two dimensional section 2 b subvertical dip of the fault 3 c juxtaposition of differing thicknesses and facies of equivalent formations across faults when these differences cannot be explained by normal growth faults 4 d anomalous tectonic wedging of subvertical basinal ductile sediments in the core of the carbonate platform normal faults cutting inclined slope margins may simulate apparent horizontal displacements platform basin margins influence thin skinned thrust tectonics in that overthrust ramps may be guided by clinoforms and oblique or lateral ramps may develop along margins that are at an angle of 0 70 to the main compressional axis

eoalpine and mesoalpine tectonics in the southern alps according to structural and stratigraphic data eoalpine and mesoalpine tectonics can be clearly recognized in the southern alps and distinguished from the better known neoalpine deformation

characterisation of marine and lacustrine sediments in a drowned thermokarst embayment richards island beaufort sea canada a multidisciplinary study was made of lakes and lagoons in a thermokarst embayment on richards island canadian beaufort sea in order to investigate the response of the mackenzie delta shoreline to changing hydrological influences seismo acoustic profiles made from an amphibious vehicle combined with sediment core data show that the deepest lakes contain up to 10 m of transparent or stratified silty mud overlying the kittigazuit sand basement palynological and microfossil data from 21 surface samples allow delineation of freshwater saltmarsh tidal flat and subtidal assemblages these modern reference data allow distinction of freshwater and marine facies in the cores and subsequent interpretation of geochemical content carbon ammonia sulphate radiocarbon dates and 137 cs show that sedimentation rates have increased by an order of magnitude since drowning of the embayment during the past millennium and that most of this sediment is from the mackenzie river plume

carbonate hosted siliceous crust type mineralization of carnic alps italy austria abstract a carbonate hosted stratabound siliceous crust type sct mineralization base metal sulphides barite fluorite occurs over large areas of carnic alps and karawanken in the eastern alps it concerns a pervasively silicified lithological unit up to some tens of metres thick which caps the unconformity landscapes developed on epicontinental devonian dinantian carbonates the sct mineralization is directly overlaid by different transgressive siliciclastic sediments which range from lower carboniferous to lower permian the presence of fragments of the sct mineralization in the transgressive siliciclastic sediment bounds its whole lithological evolution within a short stratigraphic interval of lower carboniferous age selected features of the regional and lithostratigraphic setting are discussed the chemical characterisation is based on the statistical evaluation of compositional data of 581 selected samples three significant groups of elements have been distinguished 1 the hydrological and metasomatically active elements si ba f which show a strong negative correlation amongst themselves and characterise the silica saturated aqueous solutions 2 the terrigenous elements al 2 o 3 k 2 o fe tot tio 2 b be ce la nb v y zr which suggest a continental margin environment for silica deposition 3 the sulphide metals cu pb zn ni sb as hg cd which define the metalliferous signature of the sct mineralization some consistent but still debatable genetic aspects of the sct mineralization are as follow 1 silica may be supplied by illitization of clay rich basinal sediments during their diagenesis 18 o of microcrystalline quartz ranges from 18 5 and 24 6 and is very similar to 18 o of authigenic quartz deriving from diagenetic processes of illitization of clay rich basinal sequences 2 the diagenetic evolution of these sediments may trigger off the movement of silica rich marine pore waters 34 s of barite ranges from 15 5 to 19 3 with an average of 17 7 and are in good agreement with 34 s of sulphate in ocean waters of upper devonian lower carboniferous age 3 a convective hydrological system connected with sinsedimentary transtensive tectonics active in the carnic alps since the frasnian may be the transport mechanism of aqueous solutions 4 a weak drop in ph in the dominant carbonate environment represents the conditions for silica precipitation sct mineralization showing persistent independent and distinct characters occurs over large areas also in other sites of the alpine belt and outside italy and austria therefore it points to important markers for some sedimentary sequences as well as to a worldwide significant cyclic metallogenic event it represents a new ore deposit type within the carbonate hosted mineralization

stable isotope composition of f pb zn mineralization in the valle de tena spanish central pyrenees this study dealing with the stable isotope c o and s geochemistry of f pb zn deposits is part of an integrated metallogenetic study of the valle de tena central pyrenees the objective of this work is to place constraints on the origin of the mineralizing solutions and sources of sulphur and metals two types of mineralization have been identified 1 stratabound ores linked to an emersion episode that occurred during the namurian to westfalian transition portalet mines and 2 low temperature hydrothermal veins related to post hercynian extensional tectonics yenefrito and lanuza mines and tebarray occurrences stable isotopic study indicates that the source of mineralizing components was the local upper palaeozoic country rocks in the case of the stratabound portalet deposit biogenic sulphides and connate waters modified by some meteoric input through a karst surface in the case of vein deposits stable isotopes point toward either modified connate water or marine water as the mineralizing fluid with sulphur and metals derived from a mixture in different proportions of regional igneous and meta sedimentary country rocks

acoustic properties of fine grained sediments from emerald basin toward an inversion for physical properties using the biot stoll model acoustic data from two long cores comprising marine clays and silts taken from emerald basin off nova scotia are presented high resolution measurements of compressional wave velocity attenuation and power law exponent are made using ultrasonic frequencies between 100 to 1000 khz the observed values of the frequency dependence of attenuation suggest that a nonconstant q mechanism is needed to explain these data and biot stoll theory is used to model the experimental results an inversion scheme is used to constrain physical parameters in the biot stoll dispersion relation the inversion shows that there is a restricted range of permeability and grain size by assigning reasonable values for grain size in the inversion the biot stoll model predicts unique values for the permeability and frame bulk modulus that agree well with estimates made by other means

acoustic and elastic characterization of marine sediments by analysis modeling and inversion of ultrasonic p wave transmission seismograms ultrasonic p wave transmission seismograms recorded on sediment cores have been analyzed to study the acoustic and estimate the elastic properties of marine sediments from different provinces dominated by terrigenous calcareous and diatomaceous sedimentation instantaneous frequencies computed from the transmission seismograms are displayed as gray shaded images to give an acoustic overview of the lithology of each core centimeter scale variations in the ultrasonic waveforms associated with lithological changes are illustrated by wiggle traces in detail cross correlation multiple filter and spectral ratio techniques are applied to derive p wave velocities and attenuation coefficients s wave velocities and attenuation coefficients elastic moduli and permeabilities are calculated by an inversion scheme based on the biot stoll viscoelastic model together with porosity measurements p and s wave scatter diagrams are constructed to characterize different sediment types by their velocity and attenuation porosity relationships they demonstrate that terrigenous calcareous and diatomaceous sediments cover different velocity and attenuation porosity ranges in terrigenous sediments p wave velocities and attenuation coefficients decrease rapidly with increasing porosity whereas s wave velocities and shear moduli are very low calcareous sediments behave similarly at relatively higher porosities foraminifera skeletons in compositions of terrigenous mud and calcareous ooze cause a stiffening of the frame accompanied by higher shear moduli p wave velocities and attenuation coefficients in diatomaceous ooze the contribution of the shear modulus becomes increasingly important and is controlled by the opal content whereas attenuation is very low this leads to the opportunity to predict the opal content from nondestructive p wave velocity measurements at centimeter scale resolution

marine dinocysts acritarchs and less well known npp tintinnids ostracod and foraminiferal linings copepod and worm remains

tectonics of the dolomites southern alps northern italy abstract in post variscan times the dolomites underwent a number of tectonic events which may be summarized as follows permian and triassic rifting phases broke the area into n s trending basins with different degrees of subsidence a middle triassic transpressive event then deformed the region along a n70 e axis generating flower structures within the basement volcano tectonic domal uplift and subsequent caldera formation occurred at the same time as the late ladinian magmatism early jurassic rifting also controlled the subsidence which increased eastward this long period of deformation was followed by a pre neogene late cretaceous palaeogene e w ene wsw compression which generated a w vergent belt possibly equivalent to the folded foreland of the dinaric chain a 70 km e w section of the dolomites indicates shortening of at least 10 km during the neogene the dolomites as far north as the insubric lineament were the innermost part of a s vergent thrust belt the basement of the dolomites was thrust southwards along the valsugana line onto the sedimentary cover of the venetian prealps for at least 10 km this caused a regional uplift of 3 5 km the valsugana line and its backthrusts on the northern side of the central dolomites generated a 60 km wide pop up in the form of a synclinorium within which the sedimentary cover adapted itself mainly by flexural slip often forming triangle zones the shortening linked to this folding is about 5 km with neogene thrusts faulting and folding pre existing thrust planes on the north eastern side of the dolomites neogene deformation is apparently more strictly controlled by the transpressive effects of the insubric lineament and shortening of the sedimentary cover may be greater than in the central dolomites minor deformation linked to the giudicarie belt is present in the western dolomites the present structure of the dolomites is thus the result of a number of tectonic events of different significance and different strike only a 3 dimensional restoration can unravel the true structure of the dolomites

depositional evolution of a lower paleozoic portion of the southalpine domain the mt pizzul area carnic alps italy the carnic alps represent the best exposed paleozoic succession within the alpine domain being fossiliferous mostly non metamorphic and largely complete this study focuses on the area around mt pizzul because the bedrocks record well the basin dynamics and most of the units are conodont bearing our aims were to contribute to the procedure of formalization of the lithostratigraphic units and to understand the depositional and deformation history of the study area the area has been mapped tectonic overprint constrained and the successions described measured and dated as a second order aim we discuss these data to infer the relations with other parts of the carnic basin and to recognize the global controls on sedimentation the depositional evolution can be sketched as follows pre hirnantian ramp type margin hirnantian glacioeustatic related deposits and unconformity pelagic deposition in a ramp type margin p idoli eifelian slope formation and differentiation in buildup foreslope and pelagic environments eifelian frasnian transgression and reef drowning frasnian visean probable subaerial exposures likely during in uppermost famennian and visean times and turbidite deposition visean global controls or deposits suggesting a global control are documented including the boda event the hirnantian glaciation the middle devonian reef growth the kacak event and the high frequency sea level fluctuations around the devonian carboniferous boundary the drowning of the buildups here and elsewhere in the carnic alps started during the frasnian unlike observed globally this suggests that local tectonics lead to progressive deepening up to the transition to turbidite deposition

hydrothermal origin of late proterozoic bedded chert at gusui guangdong china petrological and geochemical evidence the late proterozoic bedded chert from gusui in guangdong province southern china is characterized by bedded laminated massive and pseudobrecciated structures the chert is depleted in tio2 al2o3 and most trace elements however it is enriched in ba as sb hg and se in al fe mn ternary diagrams it falls into the hydrothermal field factor analysis shows that many elements show up in the principal trace element factor suggesting their enrichment results from leaching of the country rock by hydrothermal solutions the chert has low ree concentrations and displays an ree pattern consistent with a hydrothermal origin it may have formed in a late proterozoic rift or extension zone developed within the yunkai continental margin back arc basin with a fault system linking it to an unknown heat source at depth

dinoflagellate cysts freshwater algae and fungal spores as salinity indicators in late quaternary cores from marmara and black seas abstract seismic profiles and mollusks have been used to suggest that from 12 500 to 7000 yr bp the black sea was an isolated freshwater lake containing potable water and implying a surface salinity of 24 and that spiniferites cruciformis spiniferites inaequalis peridinium ponticum polykrikos spp and quinquecuspis concreta characterize the lower salinity of the marmara and or black seas the core top data and correlatable down core assemblage changes in time equivalent sapropelic and brown muds show that there is no evidence for differential aerobic decay of dinocysts in the study area the main acritarchs are sigmopollis psilatum concentricystes cf c rubinus and cf acritarch 8 of traverse 1978 all of which are absent from the aegean sea and decrease in abundance with increasing salinity the first two taxa have been reported previously as freshwater species fungal remains show a similar distribution pattern to the freshwater acritarchs indicating their origin from terrestrial environments freshwater chlorococcales are almost confined to the black sea but they have rare occurrences in the aegean indicating long distance transport microforaminiferal linings are abundant in the marmara sea but are absent in deep water of the black sea in the marmara sea mid late holocene assemblages

mineralogical forms of silica and their sequence of formation in silcretes the paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the paris basin and the central massif of france the initial silica deposits are most frequently opal with varying degrees of structural organization subsequently recrystallization produced varieties of progressively better organized quartz namely fibrous quartz chalcedony and lutecite which developed from deposits of microlaminated opal and microcrystalline quartz which formed from nodular opal however even during the formation of microcrystalline quartz facies dissolution and recrystallization processes continued resulting in progressively bigger quartz crystals in the final stages of silcrete development hyaline quartz crystals grew in voids these mineral sequences are determined by the solubility of each of the component parts solubility is controlled by the purity and the degree of order of the crystalline structure the most disordered minerals are the most soluble the degree of order is controlled in part by foreign ions incorporated into the lattice but also by the degree of supersaturation of silica in solution in the system which in turn controls the rate of formation of nuclei dissolution and recrystallization processes are dependent on the percolation of dilute solutions through the silcretes the concentrations of silica and other cations in these solutions is a function of the solubility of the silica phases in the host material and of the rate of percolation thus the porosity of the host material is important rapid percolation favors dissolution but in zones with slow percolation equilibrium is attained and no transformation of existing silica phases occurs it is in the zones of intermediate porosity that progressive recrystallization of quartz occurs the effect of rate of percolation on dissolution and recrystallization explains the vertical distribution of different silica polymorphs observed in numerous silcrete duricrusts

the grid work texture of authigenic microcrystalline quartz in siliceous crust type sct mineralized horizons siliceous crust type sct formations are pervasive silicified horizons associated with fluoritebarite polymetallic sulfide mineralization in carbonate rocks they almost invariably show a peculiar grid work texture formed by microcrystalline quartz the mineralogical and textural features of several sct samples from italy spain france china and brazil were investigated by optical and electron microscopy and x ray and neutron powder diffraction the resulting 001 pole figures exhibit two main components of orientation of the crystallographic c axis with respect to the normal to the sample surface one located at about 35 and the other in the range 60 85 the observed textural features are seen in all samples and they appear to be independent of the age and geographical location of the sct formation the observed grid work texture is tentatively interpreted as forming by nucleation of quartz seeds on specific faces of surrounding crystals the observed morphology and the regularly oriented populations of crystals are consistent with simultaneous growth of quartz crystals having well defined reciprocal orientations the grid work texture of quartz in sct horizons is similar to the type reported for magadi type cherts which are commonly interpreted as formed by magadiite mediated silica precipitation processes since the genesis of sct deposits is totally unrelated to alkaline lake genetic processes it is concluded that cherts showing strongly textured patterns of microcrystalline quartz may have a different origin than magadi type deposits

petrography and diagenesis of silcrete from the kalahari basin and cape coastal zone southern africa abstract silcrete is widely distributed in southern africa and occurs both in genetic association with deeply weathered bedrock weathering profile type and as a significant component of the cenozoic kalahari beds where it has formed through silicification of a variety of continental deposits nonweathering profile type silcrete fabrics are classified into four major types gs grain supported skeletal quartz grains in contact f floating skeletal grains floating in matrix m matrix skeletal grains 5 of volume and c conglomeratic detrital component containing pebbles f and m fabric types are subdivided on the presence absence of glaebules replacement by silica of a precursor matrix can account for the development of all four fabric types but at least some gs and c fabr c silcretes have formed by passive silica precipitation in packing voids glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes but their precise origin is uncertain authigenic silica is present as megaquartz microquartz length slow and length fast chalcedony predominantly in void fills cryptocrystalline silica and opal ct depending on the silica and ionic concentrations of pore waters ph and host sediment characteristics diagnostic petrographic features including glaebules colloform structures and chalcedonic vugh fills may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types formed in humid low ph environments or nonweathering profile types formed in high ph semiarid to arid environments

full waveform ultrasonic transmission seismograms a fast new method for the determination of physical and sedimentological parameters of marine sediment cores detailed information about the sediment properties and microstructure can be provided through the analysis of digital ultrasonic p wave seismograms recorded automatically during full waveform core logging the physical parameter which predominantly affects the elastic wave propagation in water saturated sediments is the p wave attenuation coefficient the related sedimentological parameter is the grain size distribution a set of high resolution ultrasonic transmission seismograms 50 500 khz which indicate downcore variations in the grain size by their signal shape and frequency content are presented layers of coarse grained foraminiferal ooze can be identified by highly attenuated p waves whereas almost unattenuated waves are recorded in fine grained areas of nannofossil ooze color encoded pixel graphics of the seismograms and instantaneous frequencies present full waveform images of the lithology and attenuation a modified spectral difference method is introduced to determine the attenuation coefficient and its power law a kf applied to synthetic seismograms derived using a constant q model even low attenuation coefficients can be quantified a downcore analysis gives an attenuation log which ranges from 700 db m at 400 khz and a power of n 1 2 in coarse grained sands to few decibels per meter and n s 0 5 in fine grained clays a least squares fit of a second degree polynomial describes the mutual relationship between the mean grain size and the attenuation coefficient when it is used to predict the mean grain size an almost perfect coincidence with the values derived from sedimentological measurements is achieved

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

pre oceanic rifting in the alps abstract a review of mesozoic mainly earliest jurassic to mid jurassic rifting in the western and central alps normal faults with throws up to a few kilometers are moderately inclined and involve rotation of crustal blocks the amount of stretching and thinning of the crust by jurassic rifting is discussed the rifting 200 165 m y pre dates the opening of the alpine oceans by spreading and transform movements 165 110 m y