magnetization acquisition in two mesozoic red sandstones abstract paleomagnetic samples have been collected from folds and other sedimentary structures produced by soft sediment deformation and from intra formational conglomerates at several locations of the chugwater formation of wyoming and the moenkopi formation of the colorado plateau both of triassic age most load casts and other small scale folds were magnetized after deformation slump folds were magnetized before or during folding two of the four conglomerates contain clasts that were magnetized before they were ripped up these observations indicate that most fine grained sandstones are magnetized after deposition but before burial by a meter or so of sediment some sandstones and finer grained samples can be magnetized very shortly after deposition if they are consolidated and dewatered by drying samples having more complicated magnetization histories can usually be identified by poor within sample grouping of remanence directions and by the behavior of individual samples during demagnetization

palaeomagnetism and magnetostiatigraphy of triassic strata in the sangre de cristo mountains and tucumcari basin new mexico usa summary we report palaeomagnetic data and a composite magnetic polarity sequence for middle and upper triassic rocks assigned to the anton chico member of the moenkopi formation and chinle group respectively exposed along the eastern flank of the sangre de cristo mountains and in the tucumcari basin of eastern and northeastern new mexico thermal demagnetization isolates a well defined dual polarity characteristic magnetization carried in most cases by haematite and interpreted as an early acquired chemical remanent magnetization crm characteristic magnetizations from 74 palaeomagnetic sites one site one bed are used to define a magnetic polarity sequence which we correlate with previously published triassic data obtained from both marine and non marine rocks preliminary correlation suggests that the resolution of magnetostratigraphic data derived from continental strata is not necessarily of lesser quality than that from marine rocks on the basis of the magnetostratigraphic data a profound unconformity is believed to separate lower middle norian and upper norian rhaetian strata of the chinle group palaeomagnetic poles derived from selected sites in steeply dipping 85 strata for the middle triassic anisian 240 ma 50 n 121 e n 8 late carman early norian 225 ma 53 n 104 e n 16 and late norian rhaetian 208 ma 59 n 77 e n 8 are in relatively good agreement with previously published data for the moenkopi formation and chinle group and related strata in southwest north america none the less comparison with palaeomagnetic poles obtained from gently dipping or flat lying triassic strata from this study anisian 46 n 112 e n 13 late carnian 54 n 87 e n 12 and previously published triassic poles in southwest north america suggest that a modest apparent rotation not greater than about 5 affects declinations from steeply dipping rocks the distribution of palaeomagnetic poles indicates 25 angular distance of apparent polar wander between about 240 and 208 ma

speciation of arsenic in greek travertines co precipitation of arsenate with calcite abstract the western part of the chalkidiki peninsula in northern greece is a geothermally active area that contains high levels of naturally derived arsenic in its alkaline groundwaters up to 3760 g l near wells equilibration of these groundwaters with atmospheric carbon dioxide leads to the precipitation of travertines that contain very high levels of arsenic up to 913 mg kg to determine the mechanism of arsenic uptake in these travertines we analyzed two different types of travertine from this region using both bulk and micro focused x ray absorption spectroscopy xas and xas and micro focused x ray fluorescence spectroscopy xrf bulk xas showed that in all of the studied samples arsenic is present in the pentavalent oxidation state arsenate xrf analyses indicated that arsenic is closely associated with the calcite matrix and that it generally does not correlate well with iron the arsenic k edge xas spectra of all samples closely matched each other and closely resembled a reference spectrum for arsenate coprecipitated with calcite rather than adsorbed or pure calcium arsenate iron on the other hand was found to be mainly present as a constituent of clay minerals of presumably detrital origin suggesting that iron hydr oxides were not sufficiently abundant to act as major scavengers for arsenic in the chalkidiki travertines we estimated that calcite in these travertines could sequester at least 25 of aqueous arsenic in the form of as v and thus immobilize a substantial part of arsenic present in the geothermal groundwaters these results may also be relevant for other areas where geothermal groundwaters carry arsenic to the surface and possibly as well for arsenic geochemistry in other environments with co2 enriched water

post depositional alteration of titanomagnetite in a miocene sandstone south texas u s a petrographic and geochemical studies have yielded information on the time space relationships of the post depositional alteration of detrital titanomagnetite ti mt in fine to medium grained sandstone from unoriented core samples taken below the water table at depths of 30 45 m of the miocene catahoula sandstone south texas aqueous sulfide introduced from sour gas reservoirs along a growth fault into part of the catahoula shortly after deposition resulted in the replacement at the periphery of ti mt grains by iron disulfide fes 2 minerals remnants of ti mt in cores of the partly sulfidized grains show no evidence of earlier hematitic oxidation after sulfidization part of the sandstone body was invaded by oxygenated groundwaters flowing down a shallowly inclined 1 hydrologic gradient the boundary between oxidized and reduced facies is clearly defined by the distribution of ferric and ferrous iron minerals and the concentrations of mo u and se in oxidized light red strata that had not been previously subjected to sulfidic reducing conditions but that are correlative with strata containing fes 2 minerals ti mt has been partly to entirely replaced pseudomorphously by hematite to form martite the absence of hematitic alteration of ti mt in the reduced facies is strong evidence that martite in the oxidized facies formed after deposition

paleomagnetic data from triassic strata zuni uplift new mexico further evidence of large magnitude triassic apparent polar wander of north america upper carnian 225 ma strata of the bluewater creek formation of the chinle group of the colorado plateau western new mexico contain predominantly reverse polarity characteristic magnetizations of high coercivity and distributed unblocking temperature up to 695 c data from a syndepositional structure suggest acquisition of a stable remanence during or soon after deposition the mean paleomagnetic pole for 13 virtual geomagnetic poles vgp accepted of 17 sites collected is located at 55 2 n 87 5 e a95 6 7 k 39 7 the angular distance between this pole and the mean of recently published middle and late norian 210 ma poles from overlying strata is 12 5 supporting the hypothesis that significant apparent polar wander occurred during late triassic time furthermore the bluewater creek pole lies to the west 6 1 angular distance of the cratonic reference pole for carnian time supporting the hypothesis of a small colorado plateau rotation since the early mesozoic anisian strata of the moenkopi formation contain dual polarity magnetizations of high coercivity and distributed unblocking temperatures up to 690 c the mean vgp of six sites yields is located at 53 1 n 96 3 e a95 6 4 k 109 2 which is similar to previously determined middle triassic poles triassic paleomagnetic poles from southwest north america from strata in continuous stratigraphic succession both on and off the colorado plateau are not consistent with the hypothesis of a late triassic stand still of the geomagnetic pole

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

palaeoenvironment of braided fluvial systems in different tectonic realms of the triassic sherwood sandstone group uk abstract fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional compressional and strike slip settings as well as on slowly subsiding passive basin margins a major rifting phase affected nw europe during the triassic and resulted in the generation of numerous sedimentary basins in the uk much of the fill of these basins is represented by fluvial and aeolian successions of the sherwood sandstone group additionally regions that experienced slow rates of mesozoic subsidence unrelated to triassic rifting also acted as sites of accumulation of the sherwood sandstone group one well exposed example being the eastern england shelf the fluvial depositional architecture of deposits of the sherwood sandstone group of the eastern england shelf a shelf edge basin is compared with similar fluvial deposits of the st bees sandstone formation eastern irish sea basin a half graben the two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors climate sediment source delivery style differences in preserved sedimentary style principally reflect their different tectonics settings analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel fill elements that are recorded predominantly by downstream accreting sandy barforms the different tectonic settings in which the two braided fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long term preservation potential on the eastern england shelf the vertical stacking of pebbly units and the general absence of fine grained units reflect a slow rate of sediment accommodation generation 18 19 4 m myr in this shelf edge basin successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved by contrast in the eastern irish sea basin of west cumbria the rate of sediment accommodation generation was substantially greater 119 m myr such that space was available to preserve complete fluvial cycles including silty drape units that cap the channelized deposits

palaeomagnetism and magnetostratigraphy of uppermost permian strata southeast new mexico usa correlation of the permian triassic boundary in non marine environments summary continental red sandstone and siltstone rocks of the dewey lake quartermaster formation at maroon cliffs near carlsbad new mexico are characterized by two components of magnetization with partially overlapping laboratory unblocking temperature spectra both magnetizations display high coercivities 100 mt probably residing in haematite a north directed magnetization with steep positive inclination unblocks between 100 and 650 c isolating a predominantly northwest directed magnetization with shallow inclination of near uniform normal polarity and maximum unblocking temperatures of 680 c we collected samples from 24 palaeomagnetic sites i e individual beds from a 60 m thick section of flat lying strata disconformably overlying carbonate and evaporite rocks of the rustler formation the upper member of the rustler formation contains a late permian early changxingian marine invertebrate and conodont fauna of the sampled sites four yield only steep magnetizations interpreted to be recent overprints eight sites did not yield well grouped site means and were excluded from the final calculations the formation mean dec 337 7 inc 9 2 k 31 6 a 95 7 8 n 12 sites defines a palaeomagnetic pole located at 55 2 n 117 5 e in good agreement with other late permian north american cratonic poles correlation of the short polarity sequence of this section of dewey lake strata is unambiguous compared with the polarity stratigraphy of marine sections in asia and supported by isotopic age determinations on a widespread bentonite bed in dewey lake strata in west texas approximately 251 ma and fossil data for the underlying rustler formation the magnetostratigraphy is consistent with deposition of the dewey lake formation during the latest changxingian late permian stage

magnetostratigraphic correlations in two cores from the late triassic lunde formation beryl field northern north sea uk the magnetostratigraphy of two cores from the late triassic lunde formation of the northern north sea are compatible with the calibrated magnetostratigraphy for the norian stage determined by gallet et al 1993 from tethyan carbonates the polarities of the cores indicate a lower to upper norian age for core 9 13a 36 and lower to middle norian age for core 9 13a a45 the isolated characteristic magnetizations are due to a triassic detrital remanent magnetization which is partially overprinted with a drilling induced remanence of two origins the drilling induced remanences are associated with 1 the main coring procedure producing an overprint which is oriented downcore and 2 the subsample coring procedure which produces an overprint parallel to the subsample core axis and directed outwards from the main core axis the elimination of those specimens with a significant drilling induced remanence is important for the determination of the magnetostratigraphy in these cores

paleomagnetism of the moenkopi formation colorado plateau basis for long term model of acquisition of chemical remanent magnetism in red beds paleomagnetic studies of moenkopi samples from several sites on the colorado plateau including a complete 427 m section at sinbad valley western colorado reveal the following 1 the presence of both normal and reversed triassic components in many of the same samples at many localities 2 the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the sinbad valley section 3 a streaked pattern of the remanence directions in any one member in the sinbad valley section similar to that expected from polar wander 4 a lack of progressive change from the base upward and 5 the common intermediate remanence directions of weak intensity suggesting the presence of two or more nearly balanced anti parallel components throughout the formation the results of our investigation combined with previously reported petrographic and paleomagnetic studies of the moenkopi formation have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism crm acquired diagenetically over a geologically long time interval from the available data we have formulated the following model to explain how the crm was acquired through natural processes acting intermittently at geologically reasonable rates when initially deposited the moenkopi sediments consisted primarily of first cycle detritus containing a few percent of iron bearing accessory minerals such as biotite hornblende magnetite and ilmenite at some time after deposition under favorable subsurface conditions intrastratal alteration began involving breakdown of iron bearing silicate and oxide minerals partial replacement of some iron rich silicate grains by hematite martitization of magnetite growth of hematite pods and blebs in ilmenite and biotite and precipitation of crystalline hematite in both primary and secondary voids the authigenesis spanned at least two polarity events and probably continued over several tens of millions of years stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events any given lithologic unit tended to be dominated by one polarity the model given above seems to be capable of explaining not only those magnetic and petrographic features found in the moenkopi formation but also those of most other red beds

laboratory depositional and compaction caused inclination errors carried by haematite and their implications in identifying inclination error of natural remanence in red beds summary undetected depositional and or compaction caused inclination errors may result in an overestimation of tectonically caused latitudinal offset hodych buchan used a single component isothermal remanent magnetization irm acquired in a dc field at a 45 angle to the bedding to test for inclination errors in silurian red beds this approach was criticized by stamatakos et al we produced synthetic depositional and compaction caused inclination errors to test hodych and buchan s approach red bed samples collected from the eocene suweiyi formation tarim basin northwest china were disaggregated using an ultrasonic cleaner and the sediments were mixed with distilled water to make a sediment slurry the sediment slurry spontaneously separated into silt dominated and clay sized parts so deposition and compaction experiments were conducted with three categories of slurries coarse grained the fine grained and a 1 1 volume ratio mixture of the coarse and fine grained slurries during compaction clay sized sediments experienced 17 19 inclination shallowing at 58 magnetic field inclination while coarser sediments showed little laboratory compaction caused inclination error acquisition of irm coercivity spectra and unblocking temperature spectra reveal that the magnetic carrier for the fine grained sample is dominated by pigmentary haematite a deposition experiment was also conducted with the coarse grained sediments which showed a range of depositional inclination error 0 30 carried by larger high unblocking temperature coercivity particles the intermediate unblocking temperature or coercivity component carried presumably by pigmentary haematite is an accurate record of the ambient magnetic field direction our data indicate that the single component irm method of hodych buchan can be used to identify and correct for the compaction caused inclination error of the fine grained samples while it failed to detect any depositional inclination error in the coarse grained samples therefore to better quantify the inclination error in red beds it is suggested that multiple irms be measured in various directions

mesozoic evolution of exotic terranes and marginal seas western north america abstract regional terrane analysis has been combined with a global evaluation of plate kinematics to produce a new tectonic model for the mesozoic evolution of western north america and its associated marginal seas the model employs a two tiered data reliability ranking system to resolve conflicts within data sets the lower tier of the ranking system involves the assignment of a numerical rank to paleomagnetic and or paleobiogeographic data based on each data set s reliability the upper tier of the ranking system places the paleomagnetic and paleobiogeographic data in perspective by assigning a relative order of importance to different types of data the most reliable data are considered to be departure rift and arrival collision times that are tightly constrained by independent data sets evidence of subduction and or strike slip motion also ranks high in the master ranking system paleomagnetic and vertebrate paleobiogeographic data come next in the hierarchy followed by invertebrate and floral paleobiogeographic data application of this approach to a case study the baja british columbia controversy has resulted in a coherent model for the entire region results derived from the application of this modelling approach to the problem of the mesozoic assembly of western north america suggest that its evolution may be modeled using a minimum of 57 large terranes oblique convergence may have been the dominant tectonic process along the western north american margin during the mesozoic causing large scale latitudinal transport of many exotic terranes the motions of the terranes produced a number of marginal seas yukonia etc that were short lived but important components of western margin tectonics a combination of collision events side swiping episodes and changes in plate kinematics led to the initiation of several regional tectonic upheavals including the nevadan and sevier orogenies

selective destructive demagnetization another microanalytic technique in rock magnetism a microanalytical technique selective destructive demagnetization sdd enables one relatively simply and reliably to determine the three dimensional small scale distribution of magnetic properties in rocks sdd is particularly useful in studies of remanence distribution lab preparation for the method entails the progressive slicing of a core specimen into a series of discs each of which is lapped down to a one grain diameter thickness subsequently grains selected on some physical property such as size shape color or position are selectively removed from the disc by either physical drilling cutting or chemical acid leaching means comparison of magnetic measurements made prior to and after grain removal permits determination of the designated magnetic property carried by the grains data acquired thus far from several different kinds of rocks show that sdd has potential usefulness in rock magnetic studies

rock magnetic properties of multicomponent natural remanent magnetization in alluvial red beds ne spain summary an earlier study of the cyclic miocene red bed sequence of la gloria spain by krijgsman et al indicated complicated behaviour of the natural remanent magnetization nrm in parts of the section particularly close to reversal boundaries we resampled part of the section with high resolution and used extensive rock magnetic analyses to characterize the magnetic remanence carriers below a conspicuous hydromorphous layer the magnetic contributions of haematite and magnetite are approximately equal while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced this is probably related to a change in hydrological conditions the nrm is characterized by 1 a low temperature 200 360 c overprint of unknown age 2 a medium temperature 360 580 c component interpreted as the characteristic remanent magnetization chrm and 3 a high temperature haematite component 600 680 c carrying the same directions as the medium temperature component there is no perceivable delay in nrm acquisition between the medium and the high temperature components they are both regarded as primary the behaviour of the nrm seems to be similar in the cyclic lithologies the acquisition of nrm thus seems to be independent of lithology in the cyclic part of the section the higher sampling resolution yielded the detection of a new polarity zone which probably represents a geomagnetic feature according to rock magnetic properties however the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable therefore the la gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology

paleomagnetic evidence that the central block of salinia california is not a far traveled terrane new paleomagnetic results from late cretaceous 75 85 m y red beds on the central block of salinia indicate that salinia was located within 6 in latitude of its current cratonal north american position during the late cretaceous after correction for neogene san andreas fault transport the red beds formed as alluvial fan overbank deposits with hematite cement deposited directly on salinian granites in the la panza range paleomagnetic analysis shows two components of magnetization in the red beds a low blocking temperature present day overprint residing in goethite and a high blocking temperature 600 component residing in hematite the hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis the bedding corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions twenty three class i sites 95 20 have an average hematite direction with inclination 54 4 and declination 18 2 95 6 1 after structural correction these paleomagnetic data suggest that salinia resided at about 35 n latitude during the late cretaceous within 6 of its current location adjacent to cratonal north america by contrast a summary of paleomagnetic data from the peninsular ranges terrane and the sur obispo terrane which are currently outboard of salinia shows northward transport of these terranes of 12 to 22 relative to their current locations in north america since the cretaceous the offsets increase systematically away from the craton with the most outboard sur obispo terrane which is composed of accretionary prism and distal forearc material showing the largest degree of northward translation

magnetostratigraphy in pursuit of missing links since the last quadrennial report magneto stratigraphy has continued to flourish the primary issues have been 1 refinements to the geomagnetic reversal time scale grts including recalibrations correlation of biostratigraphic data to the crts and revision of the terminology 2 use of magnetostratigraphic data for estimating rates of such processes as sediment accumulation evolution and faunal migration and 3 magnetic mineralogy and the origin of remanence for convenience we have divided the publications into studies of continental and marine sediments and highlight the most exciting issues within each of these sub headings in the first section of this review we will concentrate on the development of the geomagnetic reversal time scale grts with particular emphasis on refinements from the last few years we then turn our attention to applications of magnetostratigraphy in marine and continental sequences

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

palaeomagnetism and magnetic anisotropy of carboniferous red beds from the maritime provinces of canada evidence for shallow palaeomagnetic inclinations and implications for north american apparent polar wander summary a palaeomagnetic and magnetic anisotropy study was conducted on the lower middle carboniferous maringouin and shepody red bed formations of the maritime provinces of canada to detect and correct inclination shallowing because of the shallow inclinations commonly observed in red beds and the strong dependence of north america s palaeo mesozoic apparent polar wander apw on red beds inclination shallowing may substantially affect large portions of north america s apw path hematite is the primary magnetic mineral carrier in these red beds accompanied by secondary magnetite maghemite goethite and pigmentary hematite thermal and chemical demagnetization of the shepody fm successfully isolated characteristic remanence directions of d 177 i 20 4 95 6 5 n 19 and d 177 8 i 17 7 95 6 9 n 16 respectively thermal demagnetization of the maringouin fm isolated a characteristic remanence direction of d 178 7 i 24 9 95 14 5 n 9 high field anisotropy of isothermal remanence followed by alternating field and thermal cleaning on leached samples was used to isolate the fabric of hematite individual particle anisotropy was measured directly from magnetic separates using a new technique hematite s magnetic fabric and particle anisotropy were used to apply an inclination correction our inclination corrections indicate up to 10 of inclination shallowing corresponding to corrected palaeopole positions of 27 2 n 118 3 e a95 6 2 and 27 4 n 117 2 e a95 13 1 for the shepody and maringouin formations respectively this correction corresponds to a 6 increase in colatitude for carboniferous north america which translates into approximately a 650 km change in north america s palaeogeographic position the proposed position of north america supports a pangea b type reconstruction

dual polarity early carboniferous remagnetization of the fisset brook formation cape breton island nova scotia summary red siltstones and volcanic flows of the fisset brook formation of cape breton island nova scotia were folded or tilted in two phases one of late tournaisian and the other of mid namurian age upon thermal demagnetization both rock types yield three components of magnetization herein denoted as l i and h the l component has low unblocking temperatures and a direction conforming to that of the present day geomagnetic field l is inferred to be of recent origin the intermediate i component carried by haematite is of synfolding origin directions could be obtained through vector subtraction for the demagnetization interval of 300 550 c and yield a mean of d i 160 38 k 90 8 ag5 s lo after 39 per cent of the tilt correction is applied the h component has even higher unblocking temperatures which overlap to a large degree with those of the i component but analysis of intersecting great circles appears to yield a best fit direction for h which is nearly antipodal to that of the i component h is also synfolding and yields a mean direction of d i 342 38 k 120 9 ag5 6 9 after 60 per cent of tilt correction both components are interpreted as secondary chemical remanent magnetizations of late tournaisian to early namurian age a comparison of all available carboniferous results from the craton and the northern appalachians indicates that palaeolatitudes for nova scotia changed from about 20 s to about 10 s in the interval between the early and late namurian and that they changed again from about 10 s to 0 between the late namurian and the stephanian declinations show good agreement between newfoundland nova scotia new brunswick and the craton with the exception of the meguma terrain which underwent a counterclockwise rotation with respect to the mainland in the late carboniferous as noted previously by spariosu kent keppie 1984

paleomagnetism and k ar isochron dates of early jurassic basaltic flows and dikes of atlantic canada k ar isochron dates of 193 2 189 3 and 191 2 ma are reported for the shelburne dike of nova scotia the avalon dike of newfoundland and the caraquet dike of new brunswick respectively these major tholeiitic dikes agree in radiometric date with the north mountain basalt of nova scotia and with other tholeiitic flows of the newark supergroup that have been paleontologically assigned to the hettangian however further work is needed to resolve the discrepancy between the triassic jurassic boundary date of 193 6 ma suggested by the newark supergroup flows and the boundary dates of 204 213 ma adopted in recent time scales the north mountain basalt flows at nine sites on the north shore of the bay of fundy are shown to have likely magnetized during initial cooling yielding an hettangian virtual paleopole at 66 n 72 e dp 9 dm 14 new paleomagnetic results are also reported for the caraquet and avalon dikes the virtual paleopoles from hettangian flows and dikes of atlantic canada lie on a

discrepancies between experimental observations in natural and synthetic samples abstract the evidence is reviewed by comparing the data obtained from synthetic samples with those derived from natural samples of magnetic minerals present in rocks it is shown that there are many issues which are still unresolved some suggestions are made concerning the future direction of rock magnetic research such as the need for intensified studies of natural samples the aim of this note is not to disclose anything new but rather to open up some lines of thought and draw attention to questions and ideas that may have been lurking in our minds from the very beginnings of paleomagnetism

paleomagnetism of the moenkopi and chinle formations in central new mexico implications for the north american apparent polar wander path and triassic magnetostratigraphy in central new mexico red sedimentary rocks unconformably overlying permian carbonates of the san andres formation have been correlated with the early middle triassic moenkopi and late triassic chinle formations of the colorado plateau paleomagnetic samples from triassic sections exposed on basement cored uplifts along both the east and west side of the rio grande rift near albuquerque yield upon thermal and chemical demagnetization well defined high unblocking temperature dual polarity magnetizations carried by hematite the characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding parallel magnetization polarity zonation the moenkopi and lowermost chinle formations produced paleomagnetic poles respectively at 57 6 n 100 3 e n 36 sites k 74 1 a95 2 8 and 60 8 n 88 9 e n 17 sites k 130 3 a95 3 1 these data plus previously published and additional results from the underlying permian strata suggest that portions of central new mexico have experienced a small clockwise rotation i e less than 10 similar to that of the colorado plateau with respect to the north american craton the paleomagnetic directions of the chinle formation and related strata in eastern new mexico document about 12 great circle distance of rapid apparent polar wander during mid carnian to late norian times along a track which contains other cratonic poles of similar age we present a preliminary magnetic polarity time scale for the triassic that incorporates the present new mexico data and previously published data mostly form continental red bed sequences this magnetic polarity scale provides a basic framework which can be tested with future data from triassic sections where additional biostratigraphic control exists

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

palaeogeographic implications of differential inclination shallowing in permo carboniferous sediments from the donets basin ukraine abstract we present new palaeomagnetic data from upper carboniferous and lower permian grey and red sediments from the donets basin ukraine part of the palaeozoic east european platform detailed demagnetization of these units reveals two ancient components of magnetization component b which is carried by magnetite and pigmentary haematite and a high unblocking temperature component c present only in the red beds carried by detrital haematite the b and c components both pass fold tests indicating a primary or a near primary origin for magnetizations the c component however yields palaeolatiudes that are consistently lower by up to 12 of latitude or 1330 km than those derived from the b component and we argue that this is due to significant inclination shallowing of the c component a comparison with european reference palaeomagnetic data reveals that the reference data also span a large spread of palaeolatitudes for this time and we argue that unrecognized shallowing may have crept into the reference data when based on sedimentary units a more rigorous approach to selecting reference palaeomagnetic data may well be key to resolving palaeogeographic controversies at this time

rock magnetic evidence for inclination shallowing in the early carboniferous deer lake group red beds of western newfoundland summary a paleomagnetic and rock magnetic study of the carboniferous deer lake group red beds of newfoundland was performed to detect and correct for inclination shallowing results indicate a primary remanence carried by magnetite with a mean direction of d 179 7 i 33 7 95 7 2 which corresponds to a paleopole position of 22 2 n 122 3 e a95 7 6 correcting the inclination using anisotropy of anhysteretic remanence and the measured individual particle anisotropy gives a corrected direction of d 178 8 i 50 9 95 6 3 corresponding to a paleopole position at 8 4 n 122 7 e a95 7 2 this correction is larger than that of other red beds from the maritime provinces of canada but is consistent with paleoenvironmental reconstructions placing north america in a more arid climate zone our inclination corrected results have important implications for this portion of north america s apparent polar wander path and suggest a correction is needed for other red bed derived apwps we have determined the range of flattening factors f defined as the proportionality constant between the tangents of the measured i m and field i o inclinations tan i m f tan i 0 from this study and previous inclination correction studies to estimate inclination corrections using the range of haematite f factors observed in this study to correct the neogene red bed inclinations from the vall es pened es basin ne spain yields inclinations consistent with the known geomagnetic field inclination in the neogene thus indicating that the range of f factors reported here may be used to estimate the magnitude of inclination shallowing in red beds

the pictou red beds pennsylvanian pole could phanerozoic rocks in the interior united states be remagnetized the pennsylvanian red beds of the pictou group on prince edward island canada retain a stable remanence in hematite that gives a paleomagnetic pole position at 42 n 131 e dp 2 dm 4 positive conglomerate and contact tests along with dual polarities show that the remanence is primary and of stephanian age 286 306 ma for elongated pole distributions through time bingham statistics is more suitable than fisher statistics bingham statistics shows that there is a consistent discordance in the pennsylvanian to late triassic poles from the appalachian orogenic belt and interior craton of north america this discordance is most readily explained by younger partial remagnetization or possibly complete remagnetization in many rock units especially red beds of the interior craton bingham analysis also shows that there is a significant difference between the colorado plateau and interior craton that is most readily explained by a laramide 5 5 2 9 clockwise rotation of the plateau relative to the craton

magnetic intensity variations in red beds of the lodeve basin southern france and their bearing on the magnetization acquisition process summary an investigation of small scale variations of magnetization of a red bed sequence in the permian lodbve basin southern france has been carried out by means of 512 disc shaped samples each 25 mm in diameter and about 6 mm thick it is found that magnetic intensity variations reaching an order of magnitude can occur over stratigraphic distances of only a few centimetres these fluctuations show a clear lithostratigraphic control with intensity peaks systematically located at bed tops rock magnetic experiments and petrographic observations indicate that intensity variations are not due to changes in the amount or properties of the magnetic material external causes such as geomagnetic field fluctuations or superposition of reversed and normal components can be ruled out petrographical evidence suggests that the magnetization is carried by detrital haematite grains and it is proposed that the controlling factor of intensity variations is the efficiency of alignment of these grains along the earth s magnetic field the characteristic intensity patterns with peaks near bed tops can be explained by post depositional alignment of the grains with the highest efficiency near the exposed surface grain mobility can be enhanced by mechanical perturbations bioturbations hydraulic phenomena increased water content and reduced load all these factors tend to see their effect diminish away from the surface and could therefore produce the observed magnetization pattern the proposed model points to an early acquisition of the remanence in the lodbve red beds

magnetostratigraphy paleomagnetism and remanence acquisition in the triassic chugwater formation of wyoming the magnetic reversal sequence determined at five sections of the chugwater formation can be correlated over a distance of more than 200 km a detailed study of 203 cores constituting a grid of samples taken from a normal to reverse transition zone at the red grade section shows that the transition is characterized by a fairly smooth 180 direction change accompanied by an intensity decrease in about 80 cm of section the transition occurs in the same way in all vertical columns within the grid and in other columns at the same stratigraphic level up to 80 m from the grid and appears identical to the transition reported by herrero bervera and helsley at the same lithologic horizon about 1 km from the grid thermal and chemical demagnetization of grid samples removes a secondary remanence in the general direction of the earth s present magnetic field and does not reveal the presence of normal and reverse components in the same sample the transition appears to have been magnetized during a reversal of the earth s field samples probably became magnetized in a few hundred years or less while they were close to the deposition surface paleomagnetic pole positions for all five sites are very closely clustered and there appears to be no consistent long term polar wander from bottom to top in the sections the mean pole position 46 6 n 113 5 e 95 1 88 is significantly different from pole positions for the early triassic moenkopi formation from the colorado plateau suggesting that post triassic rotation has occurred between the plateau and the basement uplifts of wyoming

direct dating of paleomagnetic results from precambrian sediments in the amazon craton evidence for grenvillian emplacement of exotic crust in se appalachians of north america abstract we apply a new diagenetic dating technique to determine the age of magnetization for precambrian sedimentary rocks in the sw amazon craton two new paleomagnetic poles are reported from the rocks of the aguapei gp red beds of the fortuna fm p lat 59 8 n p lon 155 9 e a 95 9 5 k 14 18 sites n n 128 115 q 5 and the reverse polarity mudstones of the overlying vale da promissao formation p lat 49 5 n p lon 89 3 e a 95 12 5 k 30 6 sites n n 94 80 q 4 the fortuna fm magnetization is hosted by massive interstitial hematite cement and constitutes a post depositional remanence the age of diagenesis of the red beds is well constrained by the 1149 7 ma u pb age of authigenic xenotime rims on detrital zircons determined by shrimp analysis the magnetite hosted remanence of the vale da promissao fm may be detrital in origin but the age of deposition is poorly constrained the reliable and precisely dated fortuna fm paleomagnetic pole fixes the paleogeographic position of the amazon craton near the se appalachians portion of north america at 1 15 ga these data demonstrate a mobile grenvillian link between these two cratons and support the recent identification of amazon crust in the blue ridge province region of north america

use of anisotropy to determine the origin of characteristic remanence in the siwalik red beds of northern pakistan it is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone however anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains this in turn may reflect the development of the magnetic fabric of the sediment over time so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence in this work we make a study of the anisotropy of magnetic susceptibility ams in siwalik red beds of miocene age from northern pakistan comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it a coherent magnetization furthermore it should be possible to use ams fabric information to quantify the degree of pedogenesis in these miocene soils we attempted to determine the anisotropy of isothermal remanence air but found ams to be the technique of choice because of apparent changes in coercivity during air experiments we interpret the air data as resulting from metastable domains in hematite grains which change domain state during the air experiment

measuring remanence anisotropy of hematite in red beds anisotropy of high field isothermal remanence magnetization hf air summary the potential of using high field anisotropy of isothermal remanence magnetization hf air measurements for determining the origin of natural remanent magnetization in red beds and for identifying and correcting possible red bed inclination shallowing was investigated for specimens of the carboniferous shepody formation of new brunswick and nova scotia canada the technique makes it possible for a typical paleomagnetic laboratory to measure the remanence anisotropy of high coercivity hematite high field hf air was used in conjunction with 100 mt alternating field af and 120 c thermal demagnetization to separate the contribution of hematite to the remanence anisotropy from that of magnetite maghemite and goethite respectively a 5 t impulse dc magnetic field was used for the hf air to reset the magnetic moment of high coercivity hematite so that demagnetization between air orientations was not necessary the ability of a 5 t field to reset the magnetization was tested by generating an isothermal remanent magnetization acquisition curve for hematite by using impulse dc magnetic fields up to 5 t in one orientation and followed by applying a field in the opposite direction at each step each field application was treated by 120 c heating and 100 mt af demagnetization before measurement at 5 t the difference between the magnetizations applied in opposite directions disappeared indicating that no magnetic memory persisted at this field strength we performed a validity and reproducibility test of our hf air measurement technique by measuring three specimens multiple times along two orthogonal coordinate systems the method yielded highly reproducible results and on rotating the specimen s coordinates the fabric rotated by 90 as expected showing that it is not an artifact of the technique we also measured hf air on samples that had previously been chemically demagnetized in 3n hcl to remove the secondary chemically grown pigmentary hematite the hf air fabric of leached samples is similar to that of untreated samples but shows a better defined magnetic lineation and imbrication we interpret the fabric observed for the shepody formation to be a compactional fabric that has been reoriented by strain during folding following a flexural slip model

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

palaeomagnetism of early cambrian brigus red shale eastern newfoundland abstract studies of the early cambrian brigus br red shale exposed in conception bay yielded a single stable mean remanence direction of d 265 i 58 reverse polarity before tilt correction and d 279 i 48 k 47 95 6 9 forn 7 sites 25 samples after correction for gentle tilt the corresponding north pole palaeopole position is 27 s 51 e dp dm 6 9 this magnetization is characterized by a linear vector trajectory at higher unblocking temperature t ub spectra 400 680 c the magnetization directions at the low t ub range 400 c are more variable from sample to sample although they typically have steep downward inclinations with no evidence of kiaman overprinting marginal but not statistically significant improvement k 44 77 in the grouping of site means after tilt correction suggests that the predominant univectorial westerly remanance was probably acquired after deposition based on both geological and magnetic evidence the brigus shale formation appears to be remagnetized during siluro devonian times the position of the far sided antipole br 27 n 129 w located in the vicinity of the pole for the cambrian morrison river formation mr at 20 n 146 w which is discordant from the younger palaeozoic late ordovician carboniferous poles reported for the avalon zone is fortuitous since the magnetization of br is most likely secondary the magnetization is younger than the iapetus closure date determined geologically late ordovician consequently the displacement of the avalon zone with respect to the north american craton cannot be estimated the width of the proto atlantic ocean cannot be calculated

a new inclination shallowing correction of the mauch chunk formation of pennsylvania based on high field air results implications for the carboniferous north american apw path and pangea reconstructions abstract a new magnetic anisotropy study was performed on samples of the lower carboniferous mauch chunk formation of pennsylvania these red beds had been sampled for an inclination shallowing study by tan and kodama 2002 however application of a high field anisotropy of isothermal remanence magnetization hf air technique specifically designed to measure the anisotropy of hematite provides considerably different results from those previously reported the newly measured fabric has smaller anisotropy 9 17 as opposed to 25 40 and shows a pronounced ene wsw magnetic lineation that is sub parallel to the trend of the appalachians and interpretable as a hematite intersection lineation that occurred during local nnw directed shortening the measured magnetic fabric yields a new inclination correction with a corrected paleopole that is in better agreement with recently corrected carboniferous paleopoles than the previously corrected mauch chunk paleopole defining a more consistent apw path the corrected paleopoles allow calculation of new mean early 325 ma and late 312 ma carboniferous inclination corrected paleopoles for north america which can be compared to coeval but uncorrected paleopoles from gondwana results suggest a pangea b assemblage unless inclination shallowing is considered for gondwana estimating an inclination correction for gondwana sedimentary rock derived paleopoles permits a pangea a type assemblage at higher southern latitudes than previous reconstructions which we term pangea a3

palaeomagnetism of permo triassic rocks in the iberian cordillera spain acquisition of secondary and characteristic remanence magnetostratigraphy is widely used as a tool for stratigraphical correlation but there are problems in its application to rocks which have suffered complex diagenesis particularly sandy continental red beds in such cases the complete magnetic record should be studied in relation to its depositional diagenetic and structural history to this end a detailed palaeomagnetic study has been made of permian and triassic rocks in the northern iberian cordillera spain four stratigraphical units comprising the autunian 9saxonian9 buntsandstein and muschelkalk have been studied in a single section near molina de aragon which is well documented sedimentologically autunian rocks have a characteristic remanence which is exclusively reversed and corresponds to the kiaman interval of lower permian age a pole position 49 n 167 w is consistent with previous studies which indicate the rotation of iberia in post triassic times relative to the stable european plate the saxonian facies show complex multicomponent magnetizations no polarity zonation can be resolved but individual components were acquired prior to the rotation of iberia the buntsandstein is remarkable in that much of it is completely remagnetized in a direction similar to the present local geomagnetic direction this secondary remanence is carried by fine grained hematite which has been formed as a result of carbonate dissolution and other diagenetic reactions initially these iron oxides were probably superparamagnetic and remanence was only blocked in by structural inversion of the iberian cordillera in tertiary times and after rotation of the iberian plate some of the primary remanence is preserved in some fine grained lithologies of the buntsandstein indicating that normal and reversed zones of magnetization were originally present the magnetization of the muschelkalk carbonates is also complex secondary components similar to those of the buntsandstein are present but there is evidence that primary components were mainly normal during karnian times

pre and post folding magnetizations from the early devonian snowy river volcanics and buchan caves limestone victoria summary multicomponent magnetizations of the snowy river volcanics have been resolved into pre folding dec 340 1 inc 59 2 and post folding dec 10 5 inc 72 3 elements the former is attributed to initial cooling of the volcanics while the latter is probably related to the waning stages of the deformational episode which was presumably accompanied by cooling we argue that the pole position from the pre folding magnetizations lat 74 3 s long 222 7 e represents the australian palaeopole for the early devonian a reassessment of other australian pole positions determined from rocks of about this age suggests that remagnetization during the carboniferous may be common interpretation of the pole path prior to the devonian is equivocal and awaits further study the post folding magnetizations from the volcanics are similar in stability and direction to those from the buchan caves limestone dec 27 8 inc 75 6 which appears to be completely remagnetized the age of this remagnetizing event appears to be mid carboniferous if our interpretation of the pole path for this time is correct it is noted that a much unappreciated attribute of the fold test lies with identifying not just the relative age of magnetization but also the palaeohorizontal only when the palaeohorizontal is established can the calculation of a palaeopole be justified

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

paleomagnetism of silurian ordovician sediments from the valley and ridge province northwest georgia the remanent magnetism rm for 93 specimens has been determined in 34 independently oriented samples from 20 sites at ringgold gap georgia in the valley and ridge province of the southern appalachians lithologies sampled include middle ordovician through silurian fossiliferous limestones and hematitic sandstones and siltstones the section is confined within a single thrust slice thermal demagnetization results indicate that hematite is the dominant rm carrier although detrital magnetite has also been identified optically in some specimens both normal and reversed polarities are present and the resulting paleopole is 28 n 142 e 95 7 3 this pole falls within the cluster of published lower paleozoic valley and ridge poles but rather than clarifying the distribution of poles it augments the scatter three factors are thought to contribute to this scatter 1 unresolved components of rotation possibly introduced during folding and thrust faulting which result in declination anomalies 2 true vertical axis tectonic rotations during deformation and 3 insufficient knowledge of the timing of magnetization in light of these uncertainties caution should be exercised in constructing a lower paleozoic apparent polar wander path with paleopoles from the valley and ridge province

paleomagnetism of red cherts a case study in the inuyama area central japan this study is an attempt to isolate and identify the primary magnetization in red cherts red chert samples from a middle triassic sequence in the inuyama area 35 4 n 137 0 e central japan have a multicomponent magnetization which is delineated by thermal demagnetization two of the components have high coercivity and high blocking temperatures one is carried by hematite and the other by magnetite the former was concluded to be the primary magnetization for two reasons 1 it predates intraformational folding and 2 it records geomagnetic reversals the other component which accounts for the dominant part of natural remanent magnetization was probably acquired at a time long after the deposition two paleomagnetic results were drawn from the middle triassic red chert sequence 1 the mean inclination of the samples in the sequence was 1 4 6 8 therefore the paleolatitude amounts to 0 7 3 4 in contrast to the paleolatitude expected from the paleopole of eurasian continent which is about 70 and 2 the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the middle triassic are considerably longer more than several million years than the average of those in the cenozoic

the red bed controversy revisited shape analysis of colorado plateau units suggests long magnetization times several triassic and earliest jurassic sedimentary units from the colorado plateau region have distributions of virtual geomagnetic poles vgps that are highly elongate along the path of apparent polar wander apw this suggests that the remanent magnetizations measured in these units were acquired over an extended period of time possibly approaching 35 m y and are not precisely coeval with the stratigraphic age of the rock comparison with other paleomagnetic studies shows that the observed elongation is not a general attribute of the age of the rock nor is it related to paleolatitude the rocks that yield elongate vgp distributions are dominantly red to brown mudstones and it is possible that their remanence is dominated by a slowly acquired chemical remanent magnetization as suggested by larson et al j geophys res 87 1982 1081 and other authors however several superficially similar units from the colorado plateau have nearly circular vgp distributions the process by which remanence is acquired in clastic sedimentary rocks merits further study d 2002 elsevier science b v all rights reserved

secondary fe mn oxides in minerals heavily damaged by α recoil possible implications for palaeomagnetism sub micron fe mn oxides in columbite tantalite minerals are bound to metamict domains in the host these nano oxides are secondary minerals as the metamict zones formed through accumulation of damages from recoil each of which in a small volume destroys the crystal lattice of the u and th bearing columbite tantalite host transmission electron microscope investigations demonstrate that the oxides fall in the compositional range of magnetite jacobsite iwakiite fe mn 3o4 and of hematite type minerals along the fe2o3 mn2o3 join i e minerals that are known to be major carriers of magnetic remanence measured magnetic properties of isolated columbite tantalite crystals demonstrate in some samples magnetic remanence which is bound to sub micron pseudosingle domain crystals rather than to the host thus the magnetic remanence is not related to the formation of columbite tantalite but to the crystallization of the nano oxides which occur in metamict domains of the host and therefore are secondary depending on the content and distribution of u and th the nano oxides may form diachronously within an individual and among several host crystals magnetic secondary nano oxides are not restricted to columbite tantalite minerals but may occur in other minerals where high contents of fe and mn are associated with high contents of u and th rocks that show the same spatial distribution for u and fe as for instance some red sandstones may display magnetic properties that are controlled by recoil induced growth of secondary fe oxides

paleomagnetism of the chinle and kayenta formations new mexico and arizona paleomagnetic data were obtained from 22 sites 6 10 samples site in the upper shale member of the chinle formation 43 sites in the owl rock member of the chinle formation and 35 sites in the kayenta formation thermal demagnetization and data analyses indicate that within site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization chrm site mean directions define at least four antipodal polarity zones within each member formation suggesting the chrm was acquired soon after deposition fifteen site mean virtual geomagnetic poles vgps from the upper shale member of the chinle formation yield an early norian paleomagnetic pole position of 57 4 n 87 8 e k 60 a95 5 0 eighteen site mean vgps from the owl rock member of the chinle formation yield a middle norian paleomagnetic pole position of 56 5 n 66 4 e k 183 a95 2 6 twenty three site mean vgps from the kayenta formation yield a pliensbachian pole position of 59 0 n 66 6 e k 155 a95 2 4 combined with paleomagnetic poles from the moenave formation and the shinarump member of the chinle formation these data record 30 m y of north american apparent polar wander apw within a regional stratigraphic succession during the camian and norian stages of the late triassic chinle poles progress westward during the hettangian through pliensbachian stages of the early jurassic the pattern of apw changed to an eastward progression even after correction for 4 clockwise rotation of the colorado plateau a sharp comer in the apw path j1 cusp is resolved near the pole from the hettangian sinemurian 200 ma moenave formation 59 4 n 59 2 e amongst other implications the sharp change in the apw path at the j1 cusp implies an abrupt change from counterclockwise rotation of pangea prior to 200 ma to clockwise rotation thereafter

late palaeozoic mesozoic geomagnetic reversal paths and core mantle boundary summary an analysis of transitional virtual geomagnetic poles vgps from the late permain early triassic through to the late jurassic early cretaceous shows them to be located in preferred areas which coincide with present day zones of fast seismic velocity at the core mantle boundary cmb as has been observed for late cainozoic transitional vgp paths it is therefore suggested that the similar geometries of late palaeozoic mesozoic and late cainozoic polarity transitions have been impressed by sources of the cmb which have persisted through the mesozoic and tertiary to the present

transition stratigraphy and the problem of remanence lock in times in the siwalik red beds the record of paleomagnetic polarity transitions can provide relative time information in the stratigraphic record on scales of hundreds to thousands of years transition stratigraphy is here used to determine the relative lock in times of several components of magnetization in the siwalik red beds

prefolding and premegakinking magnetizations from the devonian comerong volcanics new south wales australia and their bearing on the gondwana pole path palaeomagnetic results from the late middle to early late devonian comerong volcanics in the budawang synclinorium lachlan fold belt lfb new south wales australia satisfy a fold test and support the hypothesis that foliated parts of the lfb have been folded by megakinks during the terminal mid carboniferous orogenic phase the preferred pole position is at 76 9 s 330 7 e a95 7 2 allowing calibration of the australian pole path at around 370 ma ago an alterative pole position at 71 3 s 283 1 e same a95 takes into account a possible rotation of the megakinked terrane by 15 clockwise with respect to the australian craton although there are still problems unravelling previously published results from silurian rocks in the lfb both because of unreliability and uncertainty as to where the lfb crustal units lay in relation to the australian craton there is no geological reason to suspect any large relative movement between the lfb and the craton since the middle devonian and thus the comerong volcanics pole position is representative of the late devonian for australia the new pole requires reassessment of many other poles that have previously been accepted as representing the siluro devonian for australia and gondwanaland we present reasons for believing that the magnetization or remagnetization of many rock units e g mereenie sandstone and mulga downs group australia and the gneigura supergroup africa was later than sedimentation probably during widespread early carboniferous deformation another rock unit msissi norite africa could be of early carboniferous rather than late devonian age

magnetostratigraphy of the moyero river section north western siberia constraints on geomagnetic reversal frequency during the early palaeozoic summary we present the magnetostratigraphy of the early palaeozoic moyero river section in north western siberia the section investigated which consists mainly of reddish limestones marls and greyish limestones extends from the upper cambrian to the lower silurian palaeomagnetic analyses reveal two magnetic components one is interpreted as a remagnetization acquired during siberian trap emplacement the other is believed to be early diagenetic consistent with the established lower palaeozoic apparent polar wander path for siberia reversed polarity dominates from the upper cambrian to the middle ordovician while normal polarity dominates from the upper ordovician to the lower silurian only a few reversals which are poorly defined are observed through the section in particular during the llandeilo these data are in agreement with previous magnetostratigraphic data from the baltic platform and siberia and indicate that magnetic reversal frequency was low during the ordovician because of the time delayed process of acquisition of the magnetization carried by haematite we cannot clearly ascertain if a reversed superchron occurred during the lower to middle ordovician however our results suggest that a 15 myr long reversed interval existed during the arenig

magnetic polarity stratigraphy a series of previous review articles on paleomagnetic polarity stratigraphy has documented the growth of this field and its passage through various stages of development in which work was concentrated on young lavas young deep sea sediments from piston cores and red beds cox 1969 larson and helsley 1975 opdyke 1972 harrison 1974 irving and pulliah 1976 butler and opdyke 1979 the closely related development of the geomagnetic reversal time scale has been reviewed by ness et al 1980 at the time of the last quadrennial u s national report butler and opdyke 1979 noted that the recent development of digital spinner and cryogenic magnetometers was making it possible to measure large numbers of very weakly magnetized samples in the last four years cryogenic magnetometers have been acquired by many laboratories and studies of magnetic stratigraphy involving stepwise demagnetization of hundreds of samples have become fairly common in view of the time and effort that go into a careful study of paleomagnetic stratigraphy it seems unlikely that this technique will ever become as routine a correlation tool as biostratigraphy

paleomagnetism of proterozoic and cambrian strata adrar de mauritanie cratonic west africa four paleomagnetic poles are reported from red beds of the adrar de mauritanie cratonic west africa thirteen localities were occupied and 224 samples were collected in seven units from a middle proterozoic to early paleozoic sedimentary section two poles are reported from middle and late proterozoic strata a third pole is reported from cambrian strata the magnetizations for these reside in stable high temperature components revealed by progressive thermal demagnetization analysis to 675 c the fourth pole is interpreted to reflect a permian overprinting in the cambrian strata two cambrian ordovician units sampled contained directions too scattered to allow the calculation of any pole the pole from middle proterozoic strata 998 32 ma rb sr date on argillite derived from four localities 52 samples single polarity plots at 48 7 n 206 9 e dp 1 1 dm 2 9 the pole from late proterozoic strata 775 52 ma rb sr date derived from two localities 15 samples mixed polarity plots at 34 0 n 246 9 e dp 4 6 dm 8 6 neither has correlatives in age in the paleomagnetic data set for cratonic west africa nor are there obvious correlatives with the paleomagnetic data set from the remainder of africa the pole that we consider to reflect a cambrian direction comes from three localities 23 samples mixed polarity plotting at 0 3 s 249 3 e dp 5 7 dm 11 5 its antiparallel pole corresponds with one group of cambrian poles reported from africa but differs markedly from another group of cambrian poles the late paleozoic overprint direction obtained from three units 50 samples from four localities single polarity provides a reversed polarity pole that plots at 47 2 s 42 6 e dp 1 5 dm 3 0 which is in good agreement with permian poles reported from elsewhere in africa in spite of the large number of poles already reported from africa additional data are needed before a reliable apparent polar wandering path can be constructed for the paleozoic and proterozoic of africa

secular variation in permian red beds from dome de barrot se france the do me de barrot red beds se france were deposited during the permo carboniferous reversed superchron pcrs near the palaeo equator the natural remanent magnetisation nrm resides in detrital hematite which is very resistant against thermal demagnetisation microprobe analyses reveal the presence of titanium in hematite grains indicating the volcanic and thus detrital origin therefore the nrm is a detrital remanent magnetisation and nrm variations reflect changes in the geomagnetic field tuning of the proxy parameter magnetic susceptibility to periods of the earth orbit during permian times provides relative time control the inferred average sedimentation rate is 11 8 cm 1000 yr 31 the variation in nrm directions is similar to present day secular variation sv this suggests that sv was not suppressed during at least part of the pcrs fl 2000 elsevier science b v all rights reserved