chemical and mechanical processes during burial diagenesis of chalk an interpretation based on specific surface data of deep sea sediments burial diagenesis of chalk is a combination of mechanical compaction and chemical recrystallization as well as cementation we have predicted the characteristic trends in specific surface resulting from these processes the specific surface is normally measured by nitrogen adsorption but is here measured by image analysis of scanning electron micrographs this method concentrates on the micritic matrix alone deep sea sediments are ideally suited to the study of burial diagenesis because they accumulate in a relatively conservative tectonic setting we used material from the ontong java plateau in the pacific where a 1 km thick package of chalk facies sediments accumulated from the cretaceous to the present in the upper 200 300 m the sediment is unconsolidated carbonate ooze throughout this depth interval compaction is the principal porosity reducing agent but recrystallization has an equal or larger influence on the textural development in the chalk interval below compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization below 850 m grain bridging cementation becomes important resulting in a lithified limestone below 1100 m this interpretation is based on specific surface data alone and modifies current diagenetic models

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

origin and environment of formation of late diagenetic dolomite in cretaceous tertiary chalk north sea central graben dolomite is common in parts of the chalk group of north sea petroleum reservoirs the stratigraphic distribution microtextures and stable isotope ratios of dolomite vary both within and between four of the north sea chalk fields examined in this study the eldfisk and tor of the norwegian sector the dan field of the danish sector and the machar field of the united kingdom sector indicating that the conditions favourable for dolomitization occurred at different times and places in the north sea central graben dolomitization in all of the examined fields occurred late during diagenesis after significant compactional grain breakage dolomite precipitation in the eldfisk dan and machar fields occurred in modified sea water which had 18 o values between the assumed cretaceous tertiary seawater value 1 and 8 smow the enrichment in 18 o was probably the product of calcite recrystallization at elevated temperatures in a low water rock ratio system the timing of dolomitization varied with respect to organic diagenesis dolomite precipitation in the eldfisk field coincided with bacterial methanogenesis whereas dolomite precipitation in the machar field probably coincided with bacterial sulphate reduction the magnesium in the dolomite may have been derived from the neomorphism of high magnesium calcite to low magnesium calcite

a reappraisal of dolomite abundance and occurrence in the phanerozoic critical assessment of dolomite abundance throughout the phanerozoic suggests that most large scale dolomite has had an origin related to salinity elevated seawaters the distribution of such dolomite is not directly related to periods of major continental flooding but rather to periods when extensive peritidal sequences or large scale evaporite basins lagoons existed the development of such depositional systems is controlled largely by eustasy paleogeographic tectonic setting and paleoclimate extensive peritidal sequences are characteristic of mature passive margin carbonate platforms that formed during long periods of earth history generally devoid of major continental glaciation e g late precambrian through early paleozoic and mesozoic sea level fluctuations of relatively low amplitude during such periods allowed extensive cyclic peritidal dominated platforms to develop such platforms typically lack raised rims had depositional surfaces that stayed near sea level for tens of millions of years and show little evidence of major drops in sea level that would have exposed the shelf for long periods this would have allowed repeated replenishment of pore waters by mg rich marine fluids over very extensive supratidal surfaces also since the cycles are very thin the downward moving brines could penetrate several cycles on the inner platform causing massive dolomitization in contrast during major glacial periods e g middle to late mississippian to early permian and late cenozoic higher amplitude ice driven fluctuations in sea level led to extensive meteoric water flushing of shallow platforms but related dolomitization has been of minor significance under conditions of elevated mg ca ratio in hypersaline brines however large scale dolomitization could occur in response to the formation of extensive evaporite basins lagoons suitable climatic and paleogeographic tectonic conditions have allowed such dolomitization at various times during the phanerozoic regardless of specific styles of eustatic sea level change lack of massive dolomitization in holocene carbonates may be because neither extensive peritidal sequences nor large scale evaporite basins lagoons are prevalent in modern carbonate environments

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

dolomitization by penesaline sea water in early jurassic peritidal platform carbonates gibraltar western mediterranean peritidal carbonates of the lower jurassic liassic gibraltar limestone formation which form the main mass of the rock of gibraltar are replaced by fine and medium crystalline dolomites replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an 460 m thick platform succession the fine crystalline dolomite has d 18 o values either similar to or slightly higher than those expected from early jurassic marine dolomite and d 13 c values together with 87 sr 86 sr ratios that overlap with sea water values for that time indicating that the dolomitizing fluid was early jurassic sea water absence of massive evaporitic minerals and or evaporite solution collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation the occurrence of peritidal facies a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions salinity 72 199 the medium crystalline dolomite has some d 18 o and d 13 c values and 87 sr 86 sr ratios similar to those of early jurassic marine dolomites which indicates that ambient sea water was again a likely dolomitizing fluid however the spread of d 18 o d 13 ca nd 87 sr 86 sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow 500 m burial or that dolomitization was multistage these data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation taking earlier models into consideration it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities normal penesaline to hypersaline provided that there is a driving mechanism for fluid migration the gibraltar dolomites confirm other reports of significant early jurassic dolomitization in the western tethys carbonate platforms

broadband seismic background noise at temporary seismic stations observed on a regional scale in the southwestern united states background noise power spectral density psd estimates for 54 pass cal colorado plateau rio grande rift great plains seismic transect la ristra stations were computed using data from 1999 to 2000 at long periods 0 01 0 1 hz typical vertical noise levels are approximately 12 db higher than the nearby global seismic network gsn borehole station anmo but horizontal power spec tral density psd noise levels are approximately 30 db higher long period noise levels exhibit essentially no spatial correlation along the la ristra transect indi cating that local thermal or atmosphere driven local slab tilt is the dominant source of noise in this band between 0 1 and 0 3 hz typical noise levels are dominated by naturally occurring microseismic noise and are essentially identical to those observed at anmo at short periods 0 3 8 hz typical noise levels along the network exceed anmo levels by approximately 15 db with the highest levels corresponding to proximity to cultural noise sources no significant day night variations were observed in the microseismic band however both low and high frequency noise levels show an increase of up to 8 db in median midday versus midnight noise levels we find that the major shortcomings of these shallow passcal style temporary vaults rela tive to a gsn style borehole installation are increased susceptibility to long period horizontal 20 sec noise and to surface noise sources above approximately 2 hz although the high frequency near surface noise field is unavoidable in shallow vaults we suggest that increased understanding and mitigation of local tilt effects in shallow vaults offers the possibility of significantly improving the long period noise environment

convection in groundwater below an evaporating salt lake 2 evolution of fingers or plumes the downward convection of salt fingers or plumes developed from the unstable boundary layer of an evaporating dry salt lake is examined using a numerical model and hele shaw cell experiments in the convecting layer the early small waves evolve into fingerlike or plumelike formations the number of fingers or plumes decreasing with time owing to differential growth and or coalescence comparison of intermediate formational stages of this pattern with the pattern generated by a two dimensional numerical simulation shows good qualitative agreement however there is a significant mismatch of the growth rates at long times in the computer simulation the plume length develops approximately twice as rapidly as it does in the experimental case a simple numerical experiment independent of the salt lake boundary conditions is compared to previously published laboratory scale measures of plume development in hele shaw cells which confirms the retardation of hele shaw plumes by a factor of approximately 50 this departure is attributed to the differences in dimensionality between the hele shaw flow domain and the model domain the data indicated that leading plumes develop isolated behavior at long times and may not be adequately represented in hele shaw analog models and numerical simulation provides a more accurate simulation of field scale behavior

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

strontium isotopes as tracers of groundwater induced floods the somme case study france major ions trace elements and o h and sr isotopes have been determined for surface water and groundwater in the somme catchment northern france the present study using a coupled hydrological and geochemical stable and sr isotopes approach focuses on the interactions between ground and surface water stable water isotopes 18o 2h show that all waters have a purely local origin from precipitation without significant evaporation or water rock interaction as all points plot close to the general meteoric water line water chemistry in the different catchments shows large variations in major element contents plotting na vs cl contents and mg2 no3 k so42 sr2 concentrations reveals a slight variations in groundwater composition b agricultural input into surface waters and c some similarities between ground and surface waters suggesting the former to be responsible for the chemical signature of the dissolved river load strontium isotope ratios measured in groundwater range from 0 70769 to 0 70808 clearly higher than that of chalk matrix water 0 707337 variations of sr isotope signatures in groundwater from the chalk aquifer are significant and help identifying different groundwater bodies the different signatures in groundwater are better explained by matrix heterogeneity which can present lateral facies variations than by a strong anthropogenic influence the sr isotope compositional range in surface waters is very wide varying from 0 70775 to 0 70816 and defining a trend towards a more radiogenic sr isotope signature with high sr contents and high mg sr ratios the relationship between 87sr 86sr and the mg sr ratios allows different end members to be defined the rivers are influenced by anthropogenic disturbance even during flood periods and river water has geochemical characteristics very similar to those of groundwater indicating a strong influence from groundwater feeding the rivers

physical property changes as a monitor of pelagic carbonate diagenesis an empirically derived diagenetic model for atlantic ocean basins a new physical model of carbonate diagenesis has been derived based on physical property measurements from 81 dsdp odp deep sea drilling project ocean drilling project drill sites in the atlantic ocean changing depth gradients of porosity bulk density p wave velocity vertical and horizontal and acoustic impedance characterize five successive diagenetic stages 1 compaction of ooze 2 breakage and dissolution of fossil tests 3 formation of chalk through precipitation of calcite 4 cementation and 5 compaction of calcite crystals age depth information from the drill sites was used to calculate the average duration of these processes in atlantic ocean basins the model can predict carbonate sediment physical properties down to a burial depth of 1000 m the model is presented for both lab and in situ conditions and thus can serve as a standard for comparison with lab log and seismic measurements an acoustic impedance vs age curve derived from the model allows estimation of the initial and present day diagenetic potential of carbonate sediments and chalk reservoirs

forced folds and fractures this volume is concerned with defining the major similarities and difference between forced folds and buckle folds in order that these differences can be used to recognize the type of folding and therefore the expected fracture pattern present in regions of poor exposure or where the geologist has to rely on seismic images an understanding of the differences between the two fold types their 3d geometry spatial organization fracture patterns etc provides an invaluable tool for earth scientists concerned with assessing the possible role of folds and their associated fracture patterns in controlling fluid migration and concentration within the crust the papers presented here are grouped into 4 sections contributions in the first section describe the use of numerical analyses to investigate the formation of fractures in forced folds including compaction folds and the section contains a description of large scale compaction folding in which the associated fracturing has given rise to the development of large sandstone dykes the papers in the second section deal with the formation of forced folds as a result of normal faulting in various geological environments including along a classical graben margin and around a resurgent caldera the third section contains papers relating to forced folding in compressional and strike slip regimes the final section considers the temporal and spatial relationships between forced folds and buckle folds the formation of crustal scale folds and a method of determining the distribution of strain on any folded surface a key parameter controlling the distribution of fractures

passcal instrument performance during the tibetan plateau passive seismic experiment summary the passcal instrumentation performed very well in the tibetan plateau seismic experiment this experiment has demonstrated the viability of recording high quality broadband data at temporary sites in extreme environments it also highlights some areas where further development is needed primarily more effort is needed to develop more versatile triggering options for the passcal instruments such developments could both increase the instrument9s success at recording low magnitude teleseismic events for travel time studies and save disk space when recording local events thus further increasing the feasibility of long deployments in remote regions although the use of the passcal instrument9s calibration circuitry for sensor recentering is a valuable technique more experience is needed in the construction of sensor pads to minimize tilt problems that require site visits to relevel the seismometer such as we experienced at a few sites in the winter this may also lead to improved low frequency noise levels although signals with periods greater than 200 sec can be recovered from these sites the most exciting aspect of the data we have collected is its broad frequency content and spatial density we anticipate that this type of experiment will be increasingly useful in the study of lithospheric interactions as well as deep earth structure this data was delivered to the iris data management center in early september 1993 and is available to the community two other broadband passive seismic experiments the baikal rift experiment gao et al 1992 and the rocky mountain front experiment sheehan et al 1992 should be delivered to the iris dmc in the fall of 1993 as well temporary broadband seismic experiments represent an extension of the permanent global seismic network gsn we encourage the seismological community to incorporate these data into any research using gsn data in order to maximize the potential utility of this new mode of seismic recording

surface expression of an accommodation zone within the gulf of suez rift egypt the gebel gharamul region in the western gulf of suez offers an excellent outcrop example of the structural geometries associated with an accommodation zone termination the surface expression of the accommodation zone is dominated by a basement promontory which protrudes from the rift shoulder and underlies the junction of oblique ramps from two adjacent half grabens both of which face the gulf of suez the southern half graben represents the breakaway fault of the southwest dipping tilt block domain and the northern half graben represents the collapse of the upper plate above the northeast dipping tilt block domain basement and nubian rocks in the hanging walls above the opposing low angle detachments on either side of the accommodation zone are broken by gulf parallel synthetic normal faults and orthogonal transfer faults which act as block terminations and incrementally accommodate the large scale rotation and displacement between adjacent blocks the prerift and synrift stratigraphic succession is draped over this fault template resulting in a complex and variable distribution of structures the product of an inhomogeneous stratigraphic succession within an accommodation zone

chalk dolomitization beneath localized subsiding tertiary depressions in a marginal marine setting in the paris basin france abstract dolomitization of the late cretaceous chalk in the southeastern paris basin forms kilometer size bodies that crosscut bedding these diagenetic structures were first identified during petroleum exploration of intra chalk seismic discordances and then they were cored down to 700 m the high velocity chalk forms kilometer size hill shaped structures between 150 and 600 m depth similarly the top of the chalk shows kilometer size domes and depressions with an average amplitude of 50 m and slopes over 3 whereas the base of the chalk shows only very gentle dip two drill cores 2 km apart intersecting the seismic discordances allowed us to establish that these structures correspond to dolomitized chalk and porosity reduction detailed biostratigraphic correlations between the two drill cores show that 1 the seismic discordances are not related to the stratigraphy and 2 dolomitization of the chalk coincides with an apparent compaction of the whole unit the overlying early and middle eocene formations vary spatially between 20 and 100 m thickness the distribution of their sedimentary facies indicates that the dome and depression structures at the top of the chalk developed during the middle eocene the depression structures and localized middle eocene depocenters directly overlie the dolomitized diagenetic structures in the chalk furthermore the depressions commonly correspond to zones where sand channels are cut through the early eocene claystone down to chalk groundwater circulation within the chalk may have been linked to the tertiary sand channels such circulation might have been responsible for the dolomitization of the chalk and concurrent dissolution and or compaction leading to differential subsidence in the tertiary deposits the link between tertiary localized depocenters and dolomitization constrains the chalk diagenesis to the eocene and to nearsurface environments stable isotope compositions of the dolomites indicate that sea water of normal salinity was likely the dolomitizing fluid dolomitization was apparently triggered by recharge of the chalk groundwater with sea water entering the chalk in depression areas lastly the diagenetic processes result in sharp seismic reflectors that cut across stratigraphy these seismic reflectors first interpreted as deep intra chalk erosional surfaces do not follow any depositional stratigraphy at all consequently such structures cannot be interpreted in terms of sequence stratigraphy

origin of channel systems in the upper cretaceous chalk group of the paris basin abstract the upper cretaceous chalk group of the anglo paris basin is known to show wedging beds and channel like features which disrupt the quietly deposited pelagic chalk that covered most of nw europe in the late cretaceous two d reflection seismic data from the brie region se of paris show the presence of at least two distinct intra chalk discordant reflections a top santonian and a mid campanian reflection these reflections are in places associated with up to 120 m deep channel like structures trending preferentially n s and nw se the mid campanian reflection is also sporadically associated with a massive secondary dolomite layer the thicknesses of which may reach 110 m diagenesis does not seem to account for the formation of the discordant reflections as there is neither a one to one relationship between the dolomite and discordant reflections nor are there signs of systematic collapse of the cenozoic succession over the channel like features as a result of intra chalk dissolution both reflections correlate with indurated chalk layers and hardgrounds and represent real unconformities the mid campanian reflection is furthermore associated with a stratigraphic hiatus a submarine origin is suggested due to the uninterrupted deep marine chalk facies below and above both unconformities and the unrealistically large sea level drop of more than 200 m which would be necessary for subaerial exposure of the central paris basin during the campanian the channels are oriented parallel to the margins of the basin and important bathymetric elements which could induce erosion by slope failure are not observed the channels are thus interpreted as having formed by strong mainly slope parallel bottom currents major channeling events are common in the chalk group throughout nw europe and represent palaeoceanographic re organization of bottom currents probably driven by changes in sea level and water temperature

sedimentology geochemistry and origin of phosphatic chalks the upper cretaceous deposits of nw europe santonian lower campanian and lower maastrichtian phosphatic chalks in northern france southern england and belgium are europe s largest sedimentary phosphatc deposits the stratigraphy sediment ology petrography mineralogy and geochemistry of the lithofacies are reviewed and new data presented depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high and low productivity phosphogenic systems it is concluded that phosphatic chalks were deposited in well oxygenated current swept environments phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates reduced bulk sediment accumulation rate and an enhanced rate of bioturbation precipitation of carbonate fluorapatite francolite accompanied the bacterially mediated decomposition of organic matter occurring within centimetres of the sediment seawater interface and taking place preferentially within microbial bodies and coatings in addition to the organically derived component pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction mineralization was probably a dominantly post oxic process but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation phosphogenesis occurred predominantly on the ne margins of the anglo paris basin where shallower sea floors and suitable palaeoceanographic conditions prevailed phosphogenic episodes were limited by sea level fluctuations which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity

saline groundwater in mono basin california 1 distribution mono lake california is a perennial closed basin saline lake up to 2 km of sediments have accumulated below the lake and a well log shows that saline groundwater of concentration 18 000 ppm extends to the bottom of the basin fill we investigated the groundwater system with the variable density flow and solute transport code sutra to determine if the basin s recharge and inferred groundwater salinity are consistent with observational data steady state model predictions of the position and shape of the interface separating saline from fresh groundwater are consistent with the salinity profile derived from the spontaneous potential sp log of a geothermal well on the lake s shoreline we also inferred the basin wide saline groundwater distribution and concentration from experiments with a steady state model of the entire basin groundwater system hydrologic variations around the lake determine the position of the saline fresh groundwater interface higher recharge rates characteristic of the sierra nevada shoreline push the interface far beneath the lake the interface probably lies below the shoreline around much of the rest of the lake on the low recharge northeastern shoreline the interface top may lie outside the lake edge this positions the saline groundwater discharge zone just below the playa surface and contributes to development of salt flats on the recently exposed former lake bed simulations suggest that the basin fill permeability may not have significant anisotropy possibly owing to extensive faulting which increases vertical permeability for an anisotropic basin fill the resultant increased channeling of recharge beneath the lake readily overcomes the opposing force of the saline groundwater density and it is flushed out of the basin sediments a simple permeability representation of the basin lithology with low permeability below the lake and an anisotropic transition to higher permeability outside the lake reasonably represents the well and spring water salinity observations

high frequency seismic noise at lajitas texas a preliminary analysis of high frequency 1 to 20 hz ambient seismic noise at a very quiet site in southwest texas is presented at frequencies greater than 1 hz the displacement noise power decreases at a rate between f 4 and f 5 the ambient seismic noise is also characterized by persistent narrow band peaks f 0 2 hz of unknown origin the noise appears to be propagating at phase velocities 2 5 to 4 km sec or greater which suggests higher mode surface or body waves estimates of coherence and degree of polarization of three component array data indicate that the high frequency ambient noise is relatively well organized over distances of at least 600 to 700 m for a single component but is relatively unorganized in a three component sense a principal result of this preliminary analysis is that the use of three component high frequency 1 to 20 hz arrays at very quiet sites coupled with three component processors may result in substantial improvement of the thresholds for detection and discrimination of weak seismic events

extensional fault related folding northwestern red sea egypt the duwi and hamadat areas on the northwestern margin of the red sea rift system exhibit superb outcrop examples of kilometric scale extensional fault related folds in this area the nakheil and hamadat faults of the rift border fault system consist of a series of wnw and nw trending segments that link through breached relay ramps the fault hanging walls are characterised by a series of offset synclines with non linear axial traces sub parallel to the faults the synclines are doubly plunging and in the immediate hanging wall to the normal faults the strata dip sub parallel to the fault these folds result from along strike displacement variations on the individual fault segments together with extension related fault propagation folding as the faults propagated upwards through a highly anisotropic pre rift sedimentary section in the duwi area the axial traces of the synclines are offset and bend in the regions of relay ramps in the hamadat area the synclines associated with individual fault segments are separated by transverse hanging wall anticlines that formed where segment linkage occurred numerical simulations of extension related fault propagation folding using a trishear model produce geometries that closely match those of the duwi and hamadat folds the dimensions of these hanging wall synclines and their relationships to fault displacement variations indicate that they are formed by extensional fault propagation folding rather than by frictional drag along the fault surface

dolomitization in chalk from the north sea central graben abstract sem studies of the upper cretaceous tertiary north sea chalk have revealed minor amounts of secondary dolomite occurring mainly as cement but also involving minor small scale neomorphism of some enveloped grains the dolomite content in the studied sequences varies considerably but generally amounts to 2 to 8 weight percent the dolomite consists of limpid crystals 10 to 30 micrometers in diameter occurring as widely scattered single rhombohedra minor aggregates and interlocking rhombohedra fringing stylolites the microtextural pattern observed demonstrates considerable envelopment and partial replacement of the biogenic constituents of the chalk by dolomite minor amounts of cryptograined clay minerals pyrite and silica are also found incorporated within the dolomite crystal x ray diffraction electron probe microanalysis and sem examination indicate a slightly calcium rich dolomite composition strontium concentrations average 1600 ppm the dolomite crystals are slightly depleted in 18o and enriched in 13c in comparison to the host sediment it is believed that the dolomitization resulted from relatively early diagenetic processes in a largely closed system

faulting within the eastern baikal rift as characterized by earthquake studies earthquake relocations focal mechanism determinations waveform modeling and stress analyses using focal mechanism information are used to examine the faulting processes within the western baikal rift 95 to 108 5 results of the study show that high angle dip 70 faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with m 4 5 events like the eastern rift nearly all earthquakes occurred on faults dipping 30 suggesting that in no part of the rift is significant deformation occurring on low angle structures focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift less normal faulting rakes 60 to 120 occurs in the western rift than the eastern rift waveform modeling of the two largest earthquakes 1950 mondy and 1959 srednebaikal in the western rift gives focal depths of 10 to 15 km the 1959 event appears to have occurred along the eastern boundary fault on lake baikal just to the north of proval bay a feature created a m 7 1 earthquake in 1862 the 1950 event may have occurred along the southern boundary fault of the tunka depression stress orientation information indicates a near horizontal nw se to wnw ese striking 3 in the eastern part of the western rift

experimental models of extensional forced folds we have used single layer and multilayer clay models to study the development of forced folds above normal faults our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer parallel detachments in single layer clay models extensional forced folds are upward widening monoclines anticlinal axial surfaces dip in the same direction as underlying master normal faults and synclinal axial surfaces dip in the opposite direction of master normal faults most secondary faults are upward steepening normal faults if master normal faults are steeply dipping however many secondary normal faults become high angle reverse faults at shallow depths the propagation and linkage of secondary faults into through going normal faults terminates the development of extensional forced folds more folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping most dipping beds and secondary faults are preserved in the hanging walls of the through oing normal faults in multilayer clay models with layer parallel detachments extensional forced folds are also upward widening monoclines slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge slip on overlying detachments accommodates minor thickness changes associated with upward widening of the fold secondary faults include low angle normal faults near the anticlinal axial surface minor thrust faults near the synclinal axial surface and high angle normal faults above the detachment edge the model predicted deformation patterns are similar to those of extensional forced folds from the gulf of suez and offshore norway this similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field well and seismic data

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins

sponge meadows and glass ramps state shifts and regime change abstract glass ramps are coastal marine sedimentation regimes of siliceous biomineralization and present alternatives to carbonate or siliciclastic deposition prominent chert deposits around the world record glass ramps from the eocene jurassic permian and mississippian these events ranged from regional to continental scale and lasted from a couple million years to over ten million years this review compares established cases of glass ramp deposition to clarify the bio depositional features that distinguish glass ramps from other bedded or mounded biosiliceous deposits and from carbonate ramp regimes it also compares environmental contexts of glass ramp initiation and persistence the primary sediment on these glass ramps is disaggregated spicules of siliceous sponges the sponges formed meadows on level bottom inner and mid ramp settings and loose spicules were redistributed along the shelf carbonate bioclasts were generally present but diluted by the great abundance of spicules carbonate factories were either limited to small dense patches moved offshore or re established at a later time glass ramps initiated in a wide variety of climates and oceanographic settings but appeared to initially require partial protection from storm energy the importance of marine silicic acid for such widespread biosiliceous factories is also estimated showing that glass ramps required long term adjustments in silica supply glass ramps lack modern analogues but their ecology can be interpreted in light of stable state dynamics in other modern invertebrate communities it is suggested that siliceous sponge meadows were an alternate stable state locally on carbonate ramps occurring with greater frequency and carrying greater ecological significance than have been previously expected this model warrants additional scrutiny on sponge meadows possible stabilizing feedbacks and on the thresholds of state switching

kinematic modelling of extensional fault propagation folding abstract many studies have shown that in extensional basins discrete faulting at depth is commonly linked to more distributed deformation in particular folding at higher levels such extensional fault propagation folds are particularly common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement strain the folds are cut by faults as they propagate upwards into the cover to date however there has been little investigation into the kinematics of linked basement faulting and extensional fault propagation folding here we present a two dimensional kinematic model of linked basement faulting and fault propagation folding which is based upon trishear the model allows investigation of the influence of shear zone geometry and the rate of fault propagation upon the style of folds and the strains associated with them the evolution of linked basement faulting and folding predicted by the model is compared in detail to that observed in an analogue model the kinematic model reproduces well many of the features seen both in the analogue model and reported from outcrop and seismic studies

overview of the subsurface structural pattern of the paris basin france insights from the reprocessing and interpretation of regional seismic lines abstract the study presents the methodology used by the french geological survey brgm for the building reprocessing and interpretation of selected regional seismic lines in the paris intracratonic basin france the 14 constructed e w and n s regional transects represent a total of 2 516 km length and are based on the merge of 240 seismic single profiles recorded by petroleum operators between 1971 and 1995 the regional lines have been selected to cross the main oil fields of the paris basin as well as high potential areas for oil exploration a first difficulty was to recover the raw data necessary to build up the regional transects the signal reprocessing harmonization and merge of the single seismic lines constituent of the regional transects are then described these operations represent the cornerstone of the study we put the emphasis on the primary static corrections as the targeted structures are commonly spatially associated with large seismic velocity variations in the upper cretaceous chalk and tertiary sedimentary cover the interpreted regional transects definitely give complementary information to the existing studies which generally lack seismic and therefore structural data we give an overview of the main structural and geometrical features of the paris basin inversion structures major unconformities as well as permo carboniferous basins we also describe the structural pattern and show the close relationships between the faults geometry the faults density and the geological evolution of the paris basin we distinguish 1 few large scale polyphase faults with a variscan origin representing the first order structural frame of the paris basin 2 monophase normal faults with strike slip features representing the subsurface prolongation of cenozoic grabens cropping out in the neighbourhood 3 deep normal faults sealed by the base calcareous dogger sequence related to the permo liassic extensional tectonic regime this large scale view of the paris basin has highlighted several potential exploration targets