neoformed magnetic minerals as an indicator of moderate burial the key example of middle paleozoic sedimentary rocks west virginia in order to help unravel the thermal history of middle paleozoic sedimentary rocks in west virginia a rock magnetic study was conducted with a focus on the marcellus shale vitrinite reflectance fluid inclusions microthermometry and conodont alteration index data yield contradictory burial temperature within the range 150 250 c 302 482 f the characterization of magnetite and pyrrhotite may be used as an index to track burial temperature around 200 c 392 f low temperature and room temperature magnetic measurements were performed in order to determine the magnetic assemblage three magnetic assemblages were identified that were stratigraphically distributed the goethite and nanosized magnetite a1 assemblage is mainly found in the clinton group oriskany sandstone stratigraphic interval silurian lower devonian nanosized fraction of magnetite and probably pyrrhotite a3 assemblage essentially constitutes the marcellus shale chemung formation sequence devonian microsized pyrrhotite is the typical mineral for a2 that is only identified near the alleghenian structural front overall the rare occurrence of micron pyrrhotite in our samples suggests that the study area has not experienced burial temperatures higher than 200 c 392 f

base level buffers and buttresses a model for upstream versus downstream control on fluvial geometry and architecture within sequences abstract the effects of downstream base level control on fluvial architecture and geometry are well explored in several broadly similar sequence stratigraphic models cretaceous dakota group strata u s western interior have characteristics reflecting combined downstream and upstream base level controls that these models cannot address particularly three layers of amalgamated channel belt sandstone within this group thicken and are continuous for distances 300 km along dip that stretch the reasonable lengths for which these models are intended to apply as well architecture in up dip reaches records repeated valley scale cut and fill cycles this contrasts with equivalent strata down dip which record channel scale lateral migration with no such valley scale cycles apparent we here introduce the concept of buffers and buttresses to address these observations we assume that river longitudinal profiles are each anchored down dip to some physical barrier e g the sea strand etc that we refer to as a buttress buttress shift is considered the primary downstream control on base level profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high frequency variability in upstream base level controls e g discharge etc all these potential profiles however are bounded above by the profile of highest possible aggradation and below by the profile of maximum possible incision these upper and lower profiles are buffers and they envelop the available fluvial preservation space thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance dakota valley scale surfaces record repeated cut and fill cycles driven by up dip controls and are confined between thick stable buffers equivalent strata down dip record lateral reworking within a thinner channel scale buffer zone that was positioned by downstream controls regression exposed slopes similar to the buffer zone thus buffers were stable for long distances and durations this prompted dip extensive lateral reworking of strata into upstream valley scale and downstream channel scale sheets buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs downstream base level controls on geometry and architecture the model lends general insights into dip oriented variations in fluvial architecture production of sheet vs lens geometry total preservation volumes for fluvial systems and variations in these factors related to contrasting climatic conditions and basin physiography the model can be amended to existing sequence stratigraphic approaches in order to capture dip oriented variations in sequence architecture

geomorphic thresholds and complex response of fluvial systems some implications for sequence stratigraphy first generation sequence stratigraphic models have dealt in a very rudimentary fashion with the response of fluvial systems to eustasy a major element of presently accepted models is that rivers incise when sea level falls and aggrade during the ensuing rise well established geomorphic principles state that fluvial systems are complex process response systems that can adjust to internal and external changes in other ways besides incision and aggradation by modifying their stream patterns and channel geometries application of geomorphic principles to sequence stratigraphic models results in the following observations during eustatic lowstands rivers may adjust to lowered base levels and changes in slope by modifying channel patterns therefore not all lowstands produce type 1 sequence boundaries type 1 sequence boundaries characterized by fluvial valley incision are more likely to develop when sea level drops below the shelf slope break resulting in topological relief near the strandline in which headwardly eroding knickpoints form rates of eustatic change although rapid in a geological context are sufficiently low that geomorphic systems can easily maintain their equilibrium during eustatic changes and migrate back and forth across the shelf without major modifications finally und r conditions of relatively static sea level sequences and parasequences of the same scale in time and space as those which form during eustatic change can be deposited as the result of purely intrinsic causes and responses of a fluvial system in general eustasy controls the location of deposition and erosion but the resultant stratal geometry is controlled by sediment supply and processes acting on the sediments as the shoreline moves across the shelf in response to eustasy

a late diagenetic syn folding magnetization carried by pyrrhotite implications for paleomagnetic studies from magnetic iron sulphide bearing sediments paleomagnetic rock magnetic and sedimentary micro textural data from an early miocene mudstone sequence exposed in okhta river sakhalin russia indicate the presence of pyrrhotite and magnetite at different stratigraphic levels sites that contain only magnetite have a reversed polarity characteristic remanent magnetization chrm with a low coercivity overprint which coincides with the present day geomagnetic field direction pyrrhotite bearing sites have stable normal polarity chrms that are significantly different from the present day field direction after correction for bedding tilt the chrm data fail a reversals test however the normal polarity pyrrhotite chrm directions become antipodal to the tilt corrected magnetite chrm directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40 partial unfolding these data suggest that the pyrrhotite magnetization was acquired during folding and after lock in of the magnetite remanences electron microscope observations of polished sections indicate that fluid associated halos surround iron sulphide nodules pyrrhotite is present in randomly oriented laths in and around the nodules and the nodules do not appear to have been deformed by sediment compaction this observation is consistent with a late diagenetic origin of pyrrhotite documentation of a late diagenetic magnetization in pyrrhotite bearing sediments here and in recent studies of greigite bearing sediments suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn depositional magnetization

evolution of the ridges of midelt errachidia section in the high atlas revealed by paleomagnetic data new paleomagnetic data 43 sites from mesozoic sediments are contributed in this work verifying the presence of a pervasive syntectonic early cretaceous remagnetization in the easternmost area of the moroccan high atlas using the small circle intersection method we have calculated the characteristic remagnetization direction dec 337 3 inc 38 4 that fits with a 100 ma age according to the apparent polar wander path of africa the paleomagnetic vectors of remagnetization are used to obtain the geometry during the remagnetization stage 100ma of one of the most renowned geological cross sections of the high atlas the midelt errachidia profile the partial restoration of the cross section at 100ma allows us to determine the dips of the beds at the remagnetization stage in five structures ridges or anticlines our results indicate that the five ridges that configure the midelt errachidia profile were initiated to different degrees prior to wholesale compressive deformation during the cenozoic this configuration can be explained according to two different scenarios that we discuss in this paper transpression and diapirism the geological model obtained both at present and at 100ma indicates the existence of a mesozoic cover substantially decolled from the paleozoic basement what strongly contrasts with previously published transects of the same area

lateral variation of crustal properties from aerogeophysical data in northern brazil abstracta new method is proposed for the study of crustal azimuthal anisotropy and the lateral variation of other crustal aerogeophysical parameters using a quantitative approach the processing of a large newly acquired high resolution set of aerogeophysical data is considered using the existing radon transform and the geostatistical analysis approaches the data set includes an area of the guyana shield in northern brazil which was not included in previous surveys the area is covered by dense rain forest vegetation and thick soil layers parameterization was performed considering the possible anisotropic character of the geophysical 2d data application of the newly proposed geostatistical data processing yielded high resolution images of the lateral variation of quantitative geophysical parameters which indicate good correlation with previously determined seismic anisotropy in the area average anisotropy as measured by the radon transform and variogram analysis is scale dependent at scales gr

a magnetic solution to the mupe bay mystery abstract an outcrop of wealden beds at mupe bay uk is associated with a key piece of evidence for the timing of hydrocarbon migration in the wessex basin a conglomeratic bed contains oil stained clasts and matrix that appear different upon superficial observation conventional interpretations assign differences to the erosion and transport of oil cemented clasts by wealden rivers before their incorporation into a later stained conglomeratic bed this scenario constrains the onset of oil migration in the basin to the early cretaceous however arguments have been put forward for single phase staining magnetic information may provide new ways to examine the mupe bay record of oil migration migrating fluids such as hydrocarbons have been shown to cause chemical conditions suitable for the alteration or formation of authigenic magnetite resulting in associated chemical remanent magnetization crm magnetic characterization reveals both the matrix and clasts contain multi domain magnetite but abundant hematite only exists within the clasts hysteresis parameters show the matrix has more multi domain and likely larger magnetic grains than the clasts magnetic directions are different in the clasts and matrix supporting a two phase oil staining event moreover paleomagnetic directions for the clasts after tilt correction 82 6 n and 155 2 e are consistent with biodegradation processes in the early cretaceous consistent directions in separate clasts imply the biodegradation and magnetite formation took place following transportation and incorporation of the oil cemented clasts into the conglomerate bed magnetic directions reveal that the mupe bay matrix has a remanent magnetization corresponding to today s magnetic field confirming the matrix represents an active oil seep this study represents an unprecedented use of magnetic data to date the onset of oil migration in a basin the classic two stage oil staining scenario which constrains the onset in the wessex basin to the early cretaceous is supported by our data

the arcuate fold and thrust belt of northern taiwan results of a two stage rotation revealed from a paleomagnetic study abstract the taiwan orogen is the result of the collision of the luzon volcanic arc pushed northwestward by the philippine sea plate against the chinese continental margin since the late miocene and is still uplifting nowadays the fold and thrust belt in northern taiwan shows an arcuate shape from sw to ne the strike of the main tectonic structures changes from n 0 e to n 70 e such curvature has not yet been properly interpreted so far in this study we used paleomagnetic analyses to decipher the tectonic evolution along the fold and thrust belt in northern taiwan which reveals complex magnetization patterns both in ante folding and post folding formations we also interpret that the fold and thrust belt in northern taiwan has experienced two main rotational events a 30 60 14 clockwise rotation followed by a 30 7 counterclockwise rotation during the ages of 4 6 1 5 ma

mathematical modeling of graded river profiles numerical modeling of the longitudinal profiles of rivers at grade is accomplished using the basic equations of open channel flow sediment transport equations and empirical relations for downstream variation in flow discharge sediment discharge sediment caliber and channel width only in some cases are the computed stream profiles fit exactly by any one of the commonly supported mathematical function analogs to graded profile form exponential logarithmic or power function but in most cases any of these functions can provide a fit with a degree of error smaller than would be noted in treating field data profiles dominated by spatial change in fluid and sediment discharge are distinctly power functions while profiles dominated by sediment size reduction are not necessarily exponential in form other important controls on profile shape are the degree of downstream width change in response to increasing discharge and the general range of sediment size a dynamic model of a river s approach to grade

the magnetization of clay rich rocks in sedimentary basins low temperature experimental formation of magnetic carriers in natural samples abstract rocks are often magnetically altered during burial thus providing a useful method to study their thermo chemical history in order to improve the understanding of burial diagenetic magnetization we experimentally synthesized magnetite in natural samples lower jurassic argillites from the paris basin were heated to 150 c under a confined atmosphere of argon when a magnetic field was applied during the heating a chemical remanent magnetization was acquired rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite using anhysteretic remanent magnetization as a measure of the quantity of magnetite an increase of up to 150 after heating was observed ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3 calcite and new magnetite is sometimes completely absent in samples containing less than 0 5 calcite hysteresis measurements suggest that the new magnetite is in the pseudo single domain state scanning electron microscope studies indicate that the new magnetic carriers are associated with pre existing pyrite the magnetic transformations during the experiments involved a fluid which reacted with iron bearing and sulfur bearing minerals in ways depending on local redox conditions this study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins without any external supply from long range fluid flow

deciphering the impact of diagenesis overprint on negative δ13c excursions using rock magnetism case study of ediacaran carbonates yangjiaping section south china rock magnetism is used here to investigate the genesis of one of the puzzling negative carbon isotopic excursions of the neoproterozoic in the yangtze platform south china a detailed characterization of the magnetic mineralogy which includes low temperature and high field magnetometry and classical magnetic measurement arm irm susceptibility was therefore performed along upper doushantuo and lower dengying formations outcropping in the yangjiaping section the derived magnetic parameters show variations that can be interpreted as variations in magnetic grains size and in oxide contents they show that the magnetic content is significantly reduced in samples presenting negative 13ccalcite values we interpret this as a result of magnetite dissolution and secondary carbonate precipitation during early diagenesis bacterial sulfate reduction combined with c and o isotopic data paleomagnetic techniques thus show that the upper doushantuo lower dengying negative excursion of the yangjiaping section is largely due to diagenesis although the preservation of a genuine 13c excursion of lower magnitude from 7 down to 0 instead of down to 9 as usually considered cannot be ruled out a corrected 13ccarbonate chemostratigraphic curve is therefore proposed the unambiguous identification of a strong diagenetic component for this excursion casts doubts on the primary nature of other potentially time equivalent negative excursions of the yangtze platform and thus to its correlation to negative excursions in other cratons i e shuram excursion more generally this study illustrates the potential of magnetic mineralogy characterization a low cost time efficient and non destructive technique as screening tool for diagenetic overprints of 13c and 18o

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

channel trenching and climatic change in the southern u s great plains fifteen alluvial sequences in texas and oklahoma exhibit the same late holocene record of channel trenching at 1 ka the erosion was preceded by slow alluvial sedimentation in most stream valleys resulting in the formation of a cumulic organic rich flood plain soil previously named the copan soil the soil formed during a period of regionally moister climate as indicated by pollen spectra molluscan faunas vertebrate faunas sedimentary structures and high alluvial water tables at 1 ka the regional climate changed from moist to dry coinciding with an episode of channel incision of valley floors throughout the southern great plains channel trenching occurred simultaneously in both small and large streams in drainage basins of the arkansas red trinity brazos and colorado rivers the sequences are the first documented example of widespread holocene incision accompanied by firm evidence for a synchronous change in regional climate

geomorphological evidence of recent tilting in the central amazonia region abstract geomorphometric techniques applied to remote sensing data represent powerful tools as an aid for detecting terrain features produced by recent vertical crustal movements in a variety of landscapes in this work geomorphometric analysis of the central amazonia drainage network using shuttle radar topography mission srtm data was carried out to determine surface water loading and tectonic influence on the development of the uatuma and urubu river basins since the miocene the main objective was to detect geomorphological domains of lateral stream migration and channel sinuosity changes that could indicate recent vertical crustal movement to achieve this the transverse topographic symmetry factor and sinuosity index were applied to analyze the study area drainage network comparison of the geomorphological domains with structural field data and records of seismicity suggests that they are influenced by subsurface fault reactivation which coincides in space with faults mapped in outcrops that have activated since the miocene this interpretation is corroborated by the spatial correspondence between these faults with domain boundaries and shapes by river sinuosity change when crossing these boundaries and by a high concentration of seismic activity along one of these limits the crustal movement caused by hydrological loading can also influence such geomorphological features such is the case of lateral stream migrations of a domain in the opposite sense to the amazon river flow this may be a consequence of the more pronounced expression of hydrological loading in the negro and solimoes river confluence region

reconstruction of low temperature 100 c burial in sedimentary basins a comparison of geothermometer in the intracontinental paris basin abstract the thermal evolution of sediments in the shallowest part of sedimentary basins is challenging to investigate as most of the geothermometers are at their application limits this is typically the case of the callovian oxfordian claystones at the eastern border of the paris basin a recent 2000 m deep well penetrated the entire jurassic and triassic sedimentary series and permitted coring of sediments never reached by previous drilling in this part of the basin thanks to the use of several independent geothermometers apatite fission track thermochronology fluid inclusion microthermometry clay mineralogy and sedimentary organic matter evolution the burial history of the mesozoic series has been reconstructed modeled and calibrated using kinetic algorithms relevant to the various paleothermometers used in addition to these conventional methods an innovative approach was developed using the temperature dependence of the magnetic mineral assemblage within clay rich sediments mageval thermal modeling indicates that the callovian oxfordian and the lower triassic experienced maximum burial temperatures of 50 5 c and 90 10 c respectively i e about 25 c higher than present day this temperature offset implies the erosion of several hundred meters of sediments which depending on the thermal flux and conductivity considered most likely constituted upper cretaceous chalk deposits falling sea level resulted in the exposure and subsequent dismantling of the chalk cover at the beginning of cenozoic times the eastern border of the basin was then subjected to slow erosion and discrete tectonic deformation

partial anhysteretic remanence and its anisotropy applications and grainsize dependence the distribution of remanence coercivities in a rock sample can be obtained by differentiating alternating field af demagnetization curves of anhysteretic remanent magnetization arm however significant errors can be introduced by grain interactions and by the noise inherent in differentiation we have found that 1 this error can be significantly reduced by measuring partial arm s parm s and 2 parm patterns show a strong empirical correlation with grain size parm s are imparted by exposing a sample to a small direct magnetic field in the presence of a large decreasing af the direct field is switched on over a specified range of af s thus magnetizing a fraction of the magnetite grains in the corresponding coercivity range we have further found in both synthetic and natural sediments that measurement of the anisotropy of parm enables us to detect the preferred orientations of distinct subpopulations of magnetite particles related to the size and shape of the grains

tectonic fault control of wetland distributions in the central amazon revealed by jers 1 radar imagery abstract a mosaic of jers 1 l band synthetic aperture radar sar images was used to investigate the influence of tectonic faults on wetland distributions in 200 000 km2 of central amazon lowland 0 4 lat s 60 64 long w the geographic distribution of flooded wetland was clearly evident on the mosaic due to the unique characteristics of l band radar two distinct linear boundaries were encountered limiting the northern distribution of wetlands one north of the negro river main channel and west of the branco river oriented wnw ese and one south of the negro main channel and east of the branco oriented nw se the orientations and positions of these boundaries were consistent with the prevailing tectonic fracture pattern in the region geophysical pedological and geomorphological data supported the hypothesis that these boundaries are tectonically controlled the ecological economic and biogeochemical implications of the observed wetland distribution were considered the distributions of wetland dependent biota were expected to be severely limited north of the negro main channel fish production wetland timber yields and methane emissions were also predicted to be exceptionally low in this region

an integrated paleomagnetic rock magnetic and geochemical study of the marcellus shale in the valley and ridge province in pennsylvania and west virginia an integrated paleomagnetic rock magnetic and diagenetic study of the devonian marcellus subgroup from threefolds in the valley and ridge province in pennsylvania and west virginia indicates that the unit contains an intermediate temperature chemical remanent magnetization crm with south southeast declinations and shallow negative inclinations residing in pyrrhotite and a crm with more southerly declinations and shallow positive inclinations residing in magnetite rock magnetic results confirm the presence of pyrrhotite and magnetite tilt tests indicate that pyrrhotite and magnetite crms are syntilting to posttilting and paleopoles for both crms are similar and plot on the permian part of the apparent polar wander path for north america the marcellus subgroup has a complex paragenetic sequence which includes bitumen deposition and vein formation the magnetite and pyrrhotite crms formed in the permian probably due to burial diagenetic processes and not orogenic fluids the base of the unit contains the highest total organic content toc values and highest magnetic intensities both of which decrease up section this connection between toc and magnetic intensity suggests that the magnetite formed as a result of the maturation of the organic matter although more work is needed to test this hypothesis the pyrrhotite may have formed as a result of thermochemical sulfate reduction involving organic matter during the production of methane in this gas reservoir the similar permian crm ages may be explained by thrusting which resulted in rapid burial of the marcellus subgroup as it passed through the oil window and into the gas window quickly producing magnetite and pyrrhotite respectively

low temperature magnetic behaviour near 35 k in unmetamorphosed claystones summary there is growing evidence that the magnetic assemblage of claystones illustrated by low temperature magnetic transitions at 120 k and 35 k may be representative of the peak burial temperature in the so called oil window 60 150 c in previous studies it was proposed that this magnetic assemblage is characterized by fine grained pyrrhotite fe7s8 and magnetite fe3o4 however evidence of pyrrhotite was not so obvious and the magnetic transition near 35 k of fine grained pyrrhotite has similarities with those of siderite feco3 or rhodochrosite mnco3 here we propose some diagnostic tests to distinct 50 k behaviours by studying claystones from netherlands borehole and borneo prism that experienced different peak burial temperatures we perform magnetic susceptibility temperature dependency of sirm zfc rt sirm and field cooled hysteresis loops on cooling of rt sirm 300 k to 10 k we applied a magnetic field of 5 t to enhance neel type magnetic transition it is found that our samples can be classified in two categories based on the shape of the rt sirm curve one displaying an abrupt break in slope in the remanence at 30 k called n behaviour and the other one characterized by a progressive increase of the remanent magnetization by 80 k named p behaviour the first category contains essentially magnetite and fe mn carbonates the second one magnetite and probably iron sulphides

fault imprint in clay units magnetic fabric p wave velocity structural and mineralogical signatures abstract we investigated the evolution of the structure physical properties and fluid flow properties across a fault zone within the toarcian shale formation from the tournemire underground research laboratory france with this purpose an integrated approach was used based on the analysis of petrofabric anisotropy of magnetic susceptibility ams and of p wave velocity apv and the mineralogy of two cores intercepting the f2 fault a strike slip fault with a small vertical offset the fracture patterns indicated that the fault registered a multiphase tectonic evolution two strike slip regimes with approximate ne sw and nw se compression and a normal regime with a wnw ese direction of extension were identified most of the n s fault planes collected in tf1 core have a shallow dip with values between 5 to 45 we further evidenced a close relationship between the fault architecture and petrophysical properties a preferential nw se orientation of the k1 axes was observed that is in agreement with the extensional direction of the dextral movement of the fault within the fault core an important decrease in anisotropy was observed and indicates clay platelets rotations related to tectonic deformation a weaker anisotropic change was observed in asm1 this difference may be related to the local variability of the fault zone and suggests the need to study the characteristics of a fault zone both across its different architectural zones and along the main fracture plan finally we identified evidence of minerals alteration related to fault reactivation and fluid circulation fluid circulation enabled the transformation of siderite in magnetite a neo formation of kaolinite and a precipitation of calcite were at vicinity of the damage zone fault core boundary in tf1 while the major mineralogical changes occur in the damage zone in asm1 a higher illite smectite mixed layers and calcite content was observed in the eastern side of the fault

floodplain channel morphology and networks of the middle amazon river 1 floodplain channels are important components of river floodplain systems and are known to play a key role in hydrodynamic exchange and sediment transport the amazon floodplain exhibits complex networks of these channels and despite their potential importance to this globally important wetland system these floodplain channels are relatively unstudied the research presented here is the first systematic and detailed study of the network and morphologic characteristics of a large number of these channels in the middle reach of the central amazon river using analysis of data derived from landsat enhanced thematic mapper plus etm mosaic and field survey our findings show that the channels are ubiquitous their width varies widely and some of their characteristics can be fitted using power laws potentially much like the self similar or fractal like behavior hypothesized for other types of fluvial networks in all 96 of the floodplain channels are not wide enough to be represented well or at all in the 90 m shuttle radar topography mission data channel depths are tied closely to the local amplitude of the passing main river flood wave p value of 0 75 except where there are local runoff inputs which results in substantially deeper channels which provide preferential flow paths across the floodplain channel networks imply that areas of the floodplain function for large parts of the flood cycle as separate hydrogeomorphic land units here termed floodplain hydrological units fhus these hypothesized fhus also have distinct spatial and pattern characteristics and it is suggested here that their differences could provide the beginnings of a framework for understanding the detailed hydrodynamics of the floodplain in particular different types of fhus have differences in flood water source which will have important implications for biogeochemical studies of the wetlands

burial claystones remagnetization and some consequences for magnetostratigraphy we investigate the broad lines of magnetic mineral formation for non metamorphic claystones more particularly we focus on the formation of magnetite from c 20 day experiments aiming to reproduce burial conditions it is shown that the sole action of temperature from 50 c to 250 c leads to the formation of magnetite the neoformed magnetites carry a chemical remanent magnetization which equates at least the natural remanent magnetization we propose a schematic of burial with three magnetic windows where a chemical remanent magnetization superimposed the natural remanent magnetization these are the greigite window subsurface the magnetite window depth 2 km and the pyrrhotite window depth 6 km the formation of magnetic minerals has profound consequences for the magnetostratigraphy record we propose a conceptual model that shows that the continuous production of magnetite during burial may result in magnetozones that have no relation to the age of the sediment

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

enormous ganges brahmaputra sediment discharge during strengthened early holocene monsoon rivers are the main source of terrigenous sediment delivered to continental margins and thus exert a major control on coastal evolution and sequence development however little is known about past changes in fluvial sediment loads despite the recognition of significant variation under changing climatic regimes in this study we present the first quantified estimate of sediment discharge for a major river system under conditions of an intensified early holocene monsoon development of the ganges brahmaputra river delta began ca 11000 yr b p when rising sea level flooded the bengal basin thereby trapping most of the river s discharge on the inner margin chronostratigraphic data from these deltaic deposits are used to calculate the rates of sediment storage on the margin which provide a minimum estimate of the river s past sediment load results reveal that 5 1012 m3 of sediment was stored in the bengal basin from ca 11000 to 7000 yr b p which corresponds to a mean load of 2 3 109 t yr in comparison modern sediment load of the ganges brahmaputra is 1 109 t yr ranking it first among the world s rivers and underscoring the significance of a two fold increase sustained over 4 k y furthermore the timing of immense discharge in the early holocene strongly suggests its relation to a stronger than present southwest monsoon in south asia similar patterns of high monsoon related sediment discharge have been noted throughout the tropics and subtropics suggesting a widespread fluviosedimentary response the potential magnitude of which is showcased by the ganges brahmaputra system

remagnetization in bituminous limestones of the neoproterozoic araras group amazon craton hydrocarbon maturation burial diagenesis or both 1 neoproterozoic carbonates of the araras group exhibit two distinct magnetic components across the same carbonate succession in a cross section between the amazon craton and the paraguay fold belt pink dolostones of the mirassol d oeste formation carry a dual polarity primary component whereas black bituminous limestones of the guia formation yield a secondary postfolding component magnetic signatures of the guia limestones such as high anhysteretic remanence magnetization saturation isothermal remanence magnetization ratios high frequency dependent magnetic susceptibility and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates unblocking temperatures suggest that the stable high temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations the different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence crm the presence of pyrrhotite as one of the main carriers of crm in these rocks and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage however hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals

chemical remanent magnetization in synthetic magnetite as a magnetic grain produced by a chemical process below its blocking temperature grows through a critical volume in the presence of a magnetic field its moment becomes blocked and it acquires a chemical remanent magnetization crm despite its importance to paleomagnetism the properties of crm and the controls on its behavior are still poorly understood we have modified techniques developed by taylor et al 1987 to synthesize magnetite at room temperature and atmospheric pressure whereby magnetite is produced by alteration of green rust under reducing conditions the period of synthesis is twenty four hours and a viscous component acquired over this amount of time can be demagnetized with alternating fields of 10 mt the average direction of the crm parallels the direction of the applied field and the intensity increases with increasing intensity of the applied field the intensity is linearly related to the applied field up to approximately 1 mt consistent with existing crm theory developed by analogy to the acquisition of thermal remanent magnetization trm above about 1 mt however the crm intensity approaches saturation at a rate substantially lower than that predicted by the trm analog theory furthermore the ratio of the crm to an anhysteretic remanence acquired in the same field strongly depends on the intensity of the growth field analogy to trm predicts this ratio to be independent of growth field both the principal eigenvector of the anisotropy of isothermal remanence air matrix becomes more aligned with growth field and the percent anisotropy increases as the intensity of the growth field increases the above observations suggest that the model of crm acquisition based on analogy to trm acquisition is incomplete we therefore propose a revised model which incorporates the increased alignment of easy axes with growth field finally the alignment of magnetic anisotropy ellipsoids of our synthetic samples with the growth field suggests a means to distinguish crm from trm in natural remanence

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

the mont terri rock laboratory a new international research project in a mesozoic shale formation in switzerland abstract in many countries argillaceous formations are being considered as potential host rocks for repositories of radioactive waste therefore in 1995 several organisations decided to start an international research project in the reconnaissance gallery of the mont terri motorway tunnel in north western switzerland in a mesozoic shale formation the opalinus clay aalenian this project is under the patronage of the swiss national hydrological and geological survey the following organisations are also partners in the project andra and ipsn france bgr germany enresa spain nagra switzerland jnc and obayashi japan sckcen belgium the aims of the project are to analyse the hydrogeological geochemical and rock mechanical properties of an argillaceous formation the changes of these properties induced by the excavation of galleries and to evaluate and improve appropriate investigation techniques in january 1996 eight niches were excavated for the project and 15 experiments were started in boreholes of no more than 30 m in length in the winter of 1997 1998 a new research gallery was excavated to host further experiments different excavation drilling and investigation techniques have been tested and improved significant results of the different experiments are already available some of those dealing with hydrochemistry porewater pressure measurements and the evolution of the excavation disturbed zone edz are reported in this paper

a new oil window indicator the magnetic assemblage of claystones from the baffin bay volcanic margin greenland identifying independent parameters to bracket the oil window in sedimentary basins is of first importance for petroleum exploration we investigate the magnetic assemblage of cretaceous claystones collected from the western volcanic margin of greenland these claystones experienced low grade burial 300 k properties of isothermal remanent magnetization the magnetic assemblage of immature claystones lower ro resides in goethite greigite and some magnetite the magnetic assemblage of immature to early mature claystones higher ro resides in stoichiometric magnetite and fine grained pyrrhotite by comparing these results with those from claystones from the paris basin france and the jura mountains switzerland we suggest the existence of similar mechanisms for the formation of magnetic minerals during burial if confirmed by further studies the magnetic assemblage of claystones may be diagnostic of oil windows

low temperature magnetic properties of siderite and magnetite in marine sediments low temperature magnetic techniques provide useful tools to detect the presence of magnetite and pyrrhotite in sediments through identification of their low temperature transitions to determine the amount of ultrafine grained superparamagnetic material in sediments and can potentially detect the presence of certain types of magnetotactic bacteria application of these types of experiments to nannofossil chalks from beneath the barbados accretionary prism led to some unusual results which are attributed to the presence of siderite thermal demagnetization of low temperature remanence after cooling in zero field and in a 2 5 t field both displayed large remanence losses from 20 k to 40 k below 40 k the magnetization of the chalks was much higher in the field cooled experiments than in the zero field cooled experiments low temperature hysteresis experiments made after cooling in a 2 5 t field displayed offsets in magnetization parallel to the direction of the initial applied field when measured below 40 k the offset loops can be due to either an exchange anisotropy between siderite and magnetite phases in the sediments a defect moment in the siderites or a canted moment in the siderites apparent similarity between the low temperature thermal demagnetization results from these siderite bearing sediments pure siderite and pure rhodochrosite samples and the well known 34 k transition in pyrrhotite should lead to caution in identification of pyrrhotite in marine sediments based on low temperature remanence studies alone

toward a new 250 c pyrrhotite magnetite geothermometer for claystones abstract we investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation immature burial temperature 40 c of bure callovo oxfordian claystones in the basin of paris france early mature burial temperature 85 c of opalinus lower dogger claystones from the mont terri anticline in front of the jura fold belt switzerland and mature to overmature burial temperature 7 s 8 the p transition is also detected in early mature to mature opalinus and chartreuse claystones we conduct additional experimental heating of natural opalinus claystones one set of experiments is referred to as short term heating 1 h from 100 c to 200 c it is dedicated to an investigation of the effect of short lived heating processes in geology a second set of heating experiments is designed to approach burial conditions using a gold capsule in burial like experiments we heated opalinus claystones from 150 c to 250 c for several weeks under a pressure of 100 mpa in both experiments we observe a correlative diminution of the pyrrhotite signature at 35 k with increasing temperature we interpret this trend as the appearance of magnetite we derive a parameter pm from the warming curve of a saturated isothermal remanent magnetization acquired at 10 k zfc we report on a consistent evolution of pm with temperature in the range of 40 c to 250 c including natural samples heated samples at 95 c and burial like heated samples pm first increases between 40 c up to 85 c implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature for temperatures above 85 c pm decreases up to 250 c implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite pm values obtained from mature to overmature claystones from the chartreuse are lower than the pm values obtained from the burial like heated opalinus claystones suggesting that the formation of magnetite is driven by kinetics the continuous trend of the pm parameter suggests that the magnetic properties of pyrrhotite magnetite claystones can be used to infer paleo temperatures and we propose to name this geothermometer mageval

laboratory chemical remanent magnetization in a natural claystone a record of two magnetic polarities summary a record of two magnetic polarities during the acquisition by heating of a laboratory chemical remanent magnetization crm in a natural pyrite rich claystone was investigated the samples were heated for 22 hr at 250 ci na controlled magnetic field 1 0 mt under an argon atmosphere we interpret the origin of crm as the surface oxidation of pyrite to magnetite which in turn is progressively oxidized into haematite we carried out experiments under a constant polarity magnetic field and under two opposite polarities the resulting crm was measured after cooling in zero field it is parallel to the applied field and has the direction of the last polarity thermal demagnetization under an argon atmosphere isolates an unexpected lowunblocking temperature component t ub 220 c this component probably results from thermal alteration of magnetic carriers during subsequent thermal demagnetization in the onepolarity experiments thermal demagnetization of crm above 220 c isolates a well defined component parallel to the imparted field direction in the experiments with two magnetic polarities thermal demagnetization of crm reveals two components of opposite polarity the component which is parallel to the direction of the last applied field is well defined while the other component which has the polarity of the first applied field is ill defined oppositely directed components are also detected by using alternating field demagnetization

remagnetization and chemical alteration of sedimentary rocks remagnetization is a common phenomenon in rocks and developing a greater understanding of its mechanisms has several benefits acquisition of a secondary magnetization is usually tangible evidence of a diagenetic or thermal event which can be dated using palaeomagnetic techniques this is important because the timing of diagenetic and thermal events is commonly difficult to determine remagnetization can also obscure primary magnetizations and a better understanding of remagnetization could improve our ability to uncover primary magnetizations many chemical remagnetization mechanisms have been proposed including those associated with chemical alteration by a number of different fluids e g orogenic weathering mineralizing hydrocarbons and burial diagenetic processes e g clay diagenesis maturation of organic matter this book contains case studies and review articles that focus on remagnetization chemical remagnetization mechanisms and magnetic changes associated with chemical alteration by hydrocarbons

formation of magnetic minerals at hydrocarbon generation conditions in this paper we report the pyrolysis and formation of magnetic minerals in three source rock samples from the wessex basin in dorset southern england the experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks magnetic analysis of both the heated and the unheated samples at room temperature and at very low temperatures 5 k coupled with transmission electron microscopy imaging and x ray analysis revealed the formation of nanometre sized 10 nm magnetic particles that varied across the rock samples analysed but more importantly across the pyrolysis temperature range magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 c for all rock samples and then decreased at 300 c before rising again at 320 c the newly formed magnetic minerals are suggested to be primarily pyrrhotite though magnetite and greigite are also thought to be present the sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

depositional history of lower cretaceous strata in northeastern new mexico implications for regional tectonics and depositional sequences lower cretaceous strata in northeastern new mexico reflect deposition during marine transgression and regression associated with the kiowa skull creek cycle development of regionally significant unconformities can be directly related to shoreline shifts during this cycle topography related to early cretaceous tectonic activity on crystalline basement structures including the dalhart basin tucumcari basin and the intervening bravo dome also had a direct effect on early cretaceous deposition and is reflected by thickness trends in marine strata the kiowa skull creek transgression was preceded by infilling of paleovalleys represented by the long canyon and campana sandstone beds of the glencairn formation and tucumcari shale respectively the long canyon and campana sandstone beds are equivalent to the plainview formation of central colorado and together they represent an extensive drainage network developed over a correlative unconformity that is preserved throughout most of the southern western interior glencairn and tucumcari marine shales represent clastic deposition during kiowa skull creek transgression and early regression base level drop associated with kiowa skull creek regression resulted in dramatic shoreline regression and deposition of fluvial channel sandstone in the dalhart basin which maintained a low offshore gradient this produced an erosional unconformity at the base of mesa rica channel sandstone that correlates with the well documented erosional surface at the base of the muddy sandstone in central colorado the tucumcari basin which maintained a steeper offshore gradient accumulated marine deltaic mesa rica sandstone at this time marine strata in both the tucumcari and dalhart basins thin toward the bravo dome mesa rica and upper tucumcari strata represent the only extensive marginal marine lowstand deposits found on the high plains to date associated with the southern arm of the kiowa skull creek sea major mesa rica deposition was followed by transgression that infilled only those mesa rica fluvial channels that were active at that time and also that deposited marine influenced strata of the basal pajarito formation this event most likely represents onset of the greenhorn cycle in northeastern new mexico other crystalline basement structures in the northern part of the united states western interior basin were active during early cretaceous time tectonism in these areas may be related to structures in northeastern new mexico and thus may be expressions of broad scale crustal strain in the early cretaceous western interior that is undocumented by present tectonic models

continuous production of nanosized magnetite through low grade burial geological processes such as burial can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume called blocking volume in this study we designed a heating experiment for claystones obtained from the paris basin france in the 50 130 c temperature range in order to simulate 20 nm and 20 nm minerals respectively this result led us to propose a conceptual model of nucleation and growth process of magnetite during low grade burial from 2 to 4 km depth ultrafine magnetite 20 nm was predominant over single domain magnetite 20 nm for 4 km depth transposed to natural conditions our heating steps experiment suggested that claystone type rocks are remagnetized during burial for temperatures higher than 200 c the extrapolation of our results indicated that burial remagnetization due to the chemical remanent magnetization might be larger than the natural remanent magnetization 2012 american geophysical union all rights reserved

the sequence stratigraphy of fluvial depositional systems the role of floodplain sediment storage abstract the currently used model for alluvial sequence stratigraphy has been criticised miall 1991 and an alternative model is offered here in this model alluviation mainly takes place during transgressive and not highstand phases when accommodation space allows sediment storage the high capacity for some river systems to store floodplain sediments is a critical consideration the effects of climatic and intrinsic geomorphic changes during fluvial deposition are likely to greatly complicate simple base level controlled sequence models and recognising higher order fourth and fifth order cycles will be particularly difficult

late quaternary sedimentation lower colorado river gulf coastal plain of texas investigations in the lower colorado river valley gulf coastal plain of texas have resulted in the development of a spatially and temporally controlled history of changes in channel and flood plain erosional and depositional processes when combined with paleoclimatic and stratigraphic data from the upper colorado river drainage and the record of glacio eustasy in the gulf of mexico this study permits evaluation of the relative influence of different external controls on channel and flood plain behavior the development of alluvial landforms and the development of alluvial stratigraphic sequences late pleistocene and holocene alluvial deposits of the lower colorado river have been subdivided into allostratigraphic units with chronological control afforded by radiocarbon ages in the bedrock confined valley up to 10 m of late pleistocene 20 000 14 000 yr b p sediments referred to as the eagle lake alloformation ela underlie a terrace at 17 20 m above the present day channel deposition of the ela was followed by bedrock valley incision then deposition of a complex holocene valley fill referred to as the columbus bend alloformation cba columbus bend allomembers 1 and 2 cba 1 and cba 2 underlie a terrace at 12 14 m above the present day channel cba 1 was deposited 12 000 5 000 yr b p whereas cba 2 was deposited 5 000 1 000 yr b p columbus bend allomember 3 cba 3 consists of channel and flood plain deposits that represent the past 600 yr of activity allostratigraphic units within the lower colorado valley correlate with allostratigraphic units in major valley axes of the upper colorado drainage and with records of climatic and environmental change suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply basal unconformities for holocene valley fills however appear to be 1 000 2 000 yr younger in the upper colorado drainage than they are in the lower colorado valley this time transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply but conditioned by limits on rates of up stream propagation of incision through a large drainage basin by contrast unconformities within holocene valley fills document time parallel episodes of flood plain abandonment and soil formation but little additional bedrock valley cutting and indicate decreased flood magnitudes following shifts to drier climatic conditions flood plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles which altered the rate at which precipitation was transferred to stream channels as runoff during the late pleistocene through middle holocene runoff was filtered through deep upland soils floods were for the most part less flashy and contained within channel perimeters and flood plains were constructed by lateral migration without significant vertical accretion hence the ela and cba 1 contain few vertical accretion facies exposure of bedrock surfaces during the late holocene resulted in increased flood stages deep overbank flooding and construction of flood plains by vertical accretion hence cba 2 and cba 3 contain thick vertical accretion facies allostratigraphic units and bounding unconformities persist through the bedrock confined valley to the quaternary alluvial plain but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio eustatic cycle on the alluvial plain late holocene cba 2 and modern cba 3 deposited contemporaneously with the present interglacial highstand onlap and bury the ela and cba 1 which were emplaced during the last full glacial lowstand and the transgression that followed

controls on facies distribution and stratigraphic preservation in the ganges brahmaputra delta sequence abundant sediment supply and accommodation space in the bengal basin have led to the development of a major late quaternary delta sequence this sequence has formed in a tectonically active setting and represents an important example of a high energy marine and fluvial high yield continental margin deposit recent studies have detailed the delta s stratigraphy and development noting that tectonics and sediment supply control the ganges brahmaputra more significantly than in many other delta systems these ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment supply have had on deltaic processes and sequence character unique and differing stratigraphies are found within thedeltasystem suchthatfine grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding block faulting and subsidence coarse grained deposits dominate the stratigraphy under the control of high energy fluvial processes and mixed fine coarse stratigraphies are found in areas dominantly influenced by eustatic sea level change overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events e g earthquakes and rivers avulsions and long termchangesinclimate andsediment supply modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character results show that the timing and magnitude of sediment input relative to sea level rise is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence d 2002 elsevier science b v all rights reserved

paleomagnetic dating of burial diagenesis in mississippian carbonates utah 1 the objective of this study is to test models for the origin of widespread secondary magnetizations in the mississippian deseret limestone the delle phosphatic member of the deseret limestone is a source rock for hydrocarbons and modeling studies indicate that it entered the oil window in the early cretaceous during the sevier orogeny paleomagnetic and rock magnetic results from the deseret limestone and the stratigraphically equivalent chainman shale in central and western utah indicate that the units contain two ancient magnetizations residing in magnetite burial temperatures are too low for the magnetizations to be thermoviscous in origin and they are interpreted to be chemical remanent magnetizations crms fold tests from western utah indicate the presence of a prefolding triassic to jurassic crm geochemical 87sr 86sr 13c and 18o and petrographic analyses suggest that externally derived fluids did not alter these rocks this crm was acquired at the beginning of the oil window and is interpreted to be the result of burial diagenesis of organic matter a second younger crm in western central utah is apparently postfolding and is probably late cretaceous to early tertiary in age on the basis of the thermal modeling the timing overlaps with the oil window these results are consistent with a connection between organic matter maturation and remagnetization modeling of the smectite to illite transformation in the deseret limestone suggests a mean age prior to acquisition of both crms although the range for illitization overlaps with the triassic to jurassic crm the results of this study support the hypothesis that pervasive crms can be related to burial diagenetic processes in addition paleomagnetism can be used to determine the timing of such processes which can benefit hydrocarbon exploration efforts

paleomagnetism of paleozoic asphaltic deposits in southern oklahoma usa paleomagnetic measurements on asphaltic samples from two formations in southern oklahoma have been performed a bioclastic unit from the boggy formation known as the buckhorn asphalt exhibited a stable characteristic remanent moment rm after a f demagnetization between 5 20 mt we infer from our data that very fine possibly authigenic magnetite like that shown to have a genetic relationship with the migration through rocks of hydrocarbons elmore et al 1987 is the primary rm carrier in these samples the tilt corrected paleopole for the buckhorn asphalt 121 9 e 43 5n p 1 3 m 2 3 falls on the early permian apparent polar wander path for north america of irving and irving 1982 using a 30 ma window 270 280 ma because the boggy form ation containing the buckhorn asphalt was depos ited during the late pennsylvanian desmoinesian we interpret the data to indicate magnetization during minor uplift in the early permian of the arbuckle mountain region the rm appears to have been acquired at this time probably as the result of magnetite production facilitated by the introduction time of sulfate reducing bacteria

widespread cretaceous secondary magnetization in the high atlas morocco a common origin for the cretaceous remagnetizations in the western tethys in this paper we present the first palaeomagnetic data 51 sites for mesozoic lower middle jurassic sediments of the moroccan central high atlas and address the study of a widespread remagnetization the remagnetization is characterized by a very stable component with systematic normal polarity carried by magnetite the relationship between the magnetic properties and location within the basin suggests that the acquisition of the remagnetization is controlled by basin geometry fold tests indicate that the overprint acquisition is syn folding in some structures but clearly predates the tertiary compressional stage using the small circle intersection method we have calculated the remagnetization direction d 336 4 i 29 2 comparison with the global apparent polar wander path indicates that the remagnetization was acquired during the late cretaceous probably cenomanian considering both the basinal confinement of remagnetization and the connection with other remagnetization events in the western tethys we propose a scenario explaining widespread remagnetizations in the region concerning both basin scale conditions mainly related to sediment thickness and a regional scale thermal event acting as catalyst of remagnetizations in those sedimentary basins that satisfy the basin scale conditions

effective timescales of coupling within fluvial systems this paper presents a review of the coupling concept in fluvial geomorphology based mainly on previously published work coupling mechanisms link the components of the fluvial system controlling sediment transport down the system and the propagation of the effects of base level change up the system they can be viewed at several scales at the local scale involving within hillslope coupling hillslope to channel coupling and within channels tributary junction and reach to reach coupling at larger scales coupling can be considered as zonal coupling between major zones of the system or as regional coupling relating to complete drainage basins these trends are illustrated particularly by the examples of hillslope to channel coupling in the howgill fells northwest england badland systems in southeast spain alluvial fans in spain usa and uae and base level induced dissection of neogene sedimentary basins in southeast spain as the spatial scales increase so do the timescales involved effective temporal scales relate to magnitude and frequency characteristics recovery time and propagation time the relative importance changing with the spatial scale for downsystem coupling at the local scale the first two are important with propagation time increasing in importance in larger systems especially in those involving upsystem coupling related to base level change the effective timescales range from the individual event with a return period of decades through decadal to century timescales for downsystem coupling to tens to hundreds of thousands of years for the basinwide response to base level change the effective timescales influence the relative importance of factors controlling landform development

the late quaternary evolution of the negro river amazon brazil implications for island and floodplain formation in large anabranching tropical systems abstract the rio negro has responded significantly in the late pleistocene and holocene to lagged environmental changes largely associated with activity during the last glacial in the amazon basin on the basis of geological structure the rio negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles no deposits of the upper pleniglacial were recognized in the field the oldest recognizable late pleistocene alluvial unit is the upper terrace of middle pleniglacial age ca 65 25 ka reach i tentatively correlated with the oldest terrace identified on the left bank of reach iii at that time the river was mainly an aggradational bed load system carrying abundant quartz sand a product of more seasonal conditions in the upper catchment the late glacial 14 10 ka is represented by a lower finer grained terrace along the upper basin reach i which was recognized in the tiquie curicuriari and vaupes rivers at that time the river carried abundant suspended load as a response to climatic changes associated with deglaciation since about 14 ka the river has behaved as a progradational system infilling in downstream series a sequence of structurally controlled sedimentary basins or compartments creating alluvial floodplains and associated anabranching channel systems reach ii was the first to be filled then reach iii both accumulating mainly sand fine deposits increase downstream in reach iii and become predominant in some anabranch islands of the distal reach the lowermost reaches of the negro v and vi have been greatly affected by a rising base level and associated backwater effect from aggradation of the amazon during late glacial and recent times reach v has acted almost entirely as a fine sediment trap the remarkable anavilhanas archipelago is the product of holocene deposition in the upper part of this sedimentary basin however suspended sediment load declined about 1 5 ka prior to the lower part of this basin becoming infilled the progradational behavior of the rio negro filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones illustrates how a large river system responses to profound changes in late quaternary base level and sediment supply the most stable equilibrium conditions have been achieved in the holocene in reaches iib and iiia where an anabranching channel and erosional relictual island system relatively efficiently convey water and sediment downstream reaches iiib and v never achieved equilibrium conditions during the holocene characterised as they are today with incomplete floodplains and open water

geomorphic and sedimentary response of rivers to tectonic deformation a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings abstract rivers are extremely sensitive to subtle changes in their grade caused by tectonic tilting as such recognition of tectonic tilting effects on rivers and their resultant sediments can be a useful tool for identifying the often cryptic warping associated with incipient and smaller scale epeirogenic deformation in both modern and ancient settings tectonic warping may result in either longitudinal parallel to floodplain orientation or lateral normal to floodplain orientation tilting of alluvial river profiles alluvial rivers may respond to deformation of longitudinal profile by 1 deflection around zones of uplift and into zones of subsidence 2 aggradation in backtilted and degradation in foretilted reaches 3 compensation of slope alteration by shifts in channel pattern 4 increase in frequency of overbank flooding for foretilted and decrease for backtilted reaches and 5 increased bedload grain size in foretilted reaches and decreased bedload grain size in backtilted reaches lateral tilting causes down tilt avulsion of streams where tilt rates are high and steady down tilt migration combing where tilt rates are lower each of the above effects may have profound impacts on lithofacies geometry and distribution that may potentially be preserved in the rock record fluvial sedimentary evidence for past tilting is traditionally based on the assumption that depositional features reminiscent of modern fluvial tectonic effects are evidence for past tectonic effects where it is closely associated with historically active structures or where non tectonic causes cannot be invoked however caution must be exercised when using these effects as criteria for past or current tectonic warping as these effects may be caused by non tectonic factors these non tectonic causes must be eliminated before tectonic interpretations are made

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

imprinting chemical remanent magnetization in claystones at 95 c abstract thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization crm we present an experimental study of crm production by heating claystones at 95 c a vertical magnetic field of 2 mt was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months solid fragments 9 to 26 g of claystones are included in a teflon holder that is placed in the oven under a controlled atmosphere newly formed grains acquire a crm and a thermoviscous magnetization tvrm both being parallel to the applied magnetic field crm is related to the amount of newly formed grains that pass the critical volume during the reaction to measure the acquired remanence the claystones are first cooled in a zero magnetic field and then measured using a 2g squid magnetometer in the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation we investigate the magnetic transformation of mont terri lower dogger claystones switzerland due to thermal imprinting at 95 c we simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones during dehydration the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0 23 0 07 ma m 1 day between 3 and 14 days the magnetic susceptibility increases by a few percent the increase of the remanence and of the magnetic susceptibility stops after 15 days mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes during rehydration the remanence increases again whilst magnetic susceptibility drops by a few percent after 20 days the remanence during rehydration follows a rate of 0 42 0 15 ma m 1 day by contrast when rehydration takes place later after 66 days the rate is much lower 0 09 0 04 ma m 1 day low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite newly formed magnetite and hematite carry the remanence we propose that magnetite is formed at the expense of pyrite hematite results from the progressive oxidation of newly formed magnetite our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature

chronology of murrumbidgee river palaeochannels on the riverine plain southeastern australia four major periods of palaeochannel activity have been identified on the murrumbidgee sector of the riverine plain of southeastern australia on the basis of stratigraphic information the channels reveal a picture of changing flow conditions during the last full glacial cycle the ages of the periods were determined from nearly 40 thermoluminescence dates on surficial fluvial and aeolian sediments these are named the coleambally phase which occurred from 105 to 80 ka the mid to latter part of oxygen isotope stage 5 the kerarbury phase from 55 to 35 ka stage 3 the gum creek phase from 35 to 25 ka late stage 3 to early stage 2 and the yanco phase from 20 to 13 ka late stage 2 the present flow regime was established by about 12 ka stage 1 the first two phases correlate with episodes of enhanced fluvial activity in northern and central australia and with reduced dust activity globally the phases in stage 2 appear to be associated with seasonal snow melt and increased peak flows in periods flanking the last glacial maximum source bordering aeolian dunes associated with the coleambally kerarbury and yanco phases were found however the tl dates show that some have undergone aeolian reworking thermoluminescence dating and fluvial stratigraphy have revealed a detailed picture of late quaternary climate and flow regime changes that has the potential to extend to identified deposits stratigraphically older than those described here

microscopy study of biologically mediated alteration of natural mid oceanic ridge basalts and magnetic implications 1 microbial communities have been shown over the last few years to be a significant component of the subseafloor crustal environment however their role in the low temperature alteration of seafloor basalts remains an open question among the diversity of microorganisms that can contribute to oceanic rock weathering sulfate reducing bacteria have been suggested based on sulfur isotope studies to be major actors previous laboratory experiments conducted on basalt samples from the juan de fuca ridge incubated with model sulfate reducing bacteria for several months showed that bacterial activity can play a role in the decrease of seafloor magnetic signal in this paper we characterized alteration features at the nanoscale in one of these basalt samples in order to better understand the mechanisms of the magnetic signal decrease for that purpose we used a combination of focused ion beam milling transmission electron microscopy and scanning transmission x ray microscopy fossilized microbial cells and phyllosilicates were evidenced at the surface of the sample within the sample alteration rims mostly composed of fe and s and measuring 100 300 nm in thickness were observed around titanomagnetites crystals that bear most of the magnetic signal in contrast these features were not observed on noninoculated control samples this study offers a detailed view of the specific mineral assemblages formed in the presence of model sulfate reducing bacteria that can be looked for in the oceanic crust these observations contribute to understand the potential role of microbes in the alteration of the oceanic crust