remagnetization and northward translation of mesozoic red chert from cedros island and the san benito islands baja california mexico samples for paleomagnetic analysis were collected from red radiolarian ribbon chert in a franciscan like subduction complex western baja terrane exposed along the western margin of the baja california peninsula a 40 m thick section of chert was sampled on cedros island and a 30 m thick and two smaller sections of chert were sampled on the san benito islands to the west a site in pillow basalt underlying the chert also was sampled on cedros island the sections of chert on cedros island and the san benito islands contain radiolaria ranging in age from late triassic to early cretaceous all of these rocks have similar normal polarity direction of magnetization indicating that they were remagnetized long after deposition results from a chert site including an intrastratal fold also indicate that the magnetization is postdepositional a significantly improved grouping 95 confidence level of directions after correction for the structural attitudes of the cedros and san benito chert sections however implies that the remagnetization predates breaking up and incorporation of the chert into the subduction complex studies of modern subduction zones imply that the attitude of the ancient subducting slab including the chert sections was near horizontal dip angles

early and late paleozoic remagnetization of the cambrian peerless formation colorado abstract the upper cambrian peerless formation in central colorado contains two secondary components of magnetization the dominant component was removed during initial af demagnetization of specimens whereas a weaker component was removed during subsequent thermal demagnetization irm acquisition experiments suggest that the dominant component has low coercivity whereas the weaker component has high coercivity the latter component has southeasterly and shallow directions and the corresponding pole position is located at 40 0 n 134 5 e the high coercivity blocking temperatures over 600 c and petrographic evidence suggest this component resides in diagenetic hematite the magnetization is interpreted as a crm acquired in the late paleozoic the low coercivity component directions fall on a great circle that passes through the modern field direction and two modes at 107 e 10 and 287 e 10 this component is interpreted as a vector sum of two antipodal crms and a modern vrm the pole position 10 n 150 e suggests acquisition in the early paleozoic and the remanence resides either in maghemite or in diagenetic magnetite

ordovician faunas of burgess shale type the renowned soft bodied faunas of the cambrian period which include the burgess shale disappear from the fossil record in the late middle cambrian after which the palaeozoic fauna dominates the disappearance of faunas of burgess shale type curtails the stratigraphic record of a number of iconic cambrian taxa one possible explanation for this loss is a major extinction but more probably it reflects the absence of preservation of similar soft bodied faunas in later periods here we report the discovery of numerous diverse soft bodied assemblages in the lower and upper fezouata formations lower ordovician of morocco which include a range of remarkable stem group morphologies normally considered characteristic of the cambrian it is clear that biotas of burgess shale type persisted after the cambrian and are preserved where suitable facies occur the fezouata biota provides a link between the burgess shale communities and the early stages of the great ordovician biodiversification event

onuphionella corusca sp nov an early cambrian type agglutinated tube from upper ordovician strata of morocco abstract a variety of unrelated organisms produce agglutinated tubes as dwelling structures and the habit has persisted since the neoproterozoic onuphionella an agglutinated tube composed of mica flakes has been found in lower cambrian strata on several palaeocontinents in this paper a new species of the genus onuphionella corusca sp nov is described from the sandbian upper ordovician first bani group of the erfoud region morocco the first bani group was formed in a storm dominated shelf environment below fair weather wave base the agglutinated tubes are found in abundance on numerous bedding surfaces possibly representing repeated colonization between storm events and the tube maker must have been a major part of the local ecosystem this is the first report of onuphionella from ordovician rocks and the first from gondwana supplementary material data tables for major oxide and trace element results are available at https doi org 10 6084 m9 figshare c 4283612

particle size selection and tube structure of the polychaete owenia fusiformis abstract agglutinated tubes of the polychaete owenia fusiformis delle chiaje and their adjacent sediment were analyzed with respect to grain size particle composition and tube structure to assess whether these parameters reflect a specific relationship between tube and adjacent sediment tubes and sediments were sampled southwest off helgoland island southeastern north sea in water depth of 35 37 m particle selection was studied using settling velocity and component analysis both methods clearly showed that o fusiformis actively selects coarse particles for tube building the ability for size selection seems to be fully developed immediately after the transition from pelagic to benthic life as considerable differences in the composition of tube and adjacent sediments occurred over the whole sampled sediment depth in both tube and surface sediments quartz was the dominating component 80 in contrast heavy minerals the second frequent component of the surface sediments were used less frequently for tube building biogenic components e g shell material echinoid fragments foraminifera occurring rarely in the sediment are nevertheless preferred as components for tube building as underlying selection criteria a combination of size and the existence of at least one flattened surface that can be attached to the tube may be assumed

thermally activated viscous remanence in some magnetite and hematite bearing dolomites a brunhes age viscous remanent magnetization vrm carried by magnetite and hematite in the el paso and montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single domain theory the samples appear to contain a substantial fraction of pseudo single domain magnetite suggesting transdomain processes may be responsible for the stable vrm a strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0 3 tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures larger grains are inferred to have lower coercivities but higher unblocking temperatures an older reverse polarity component is also observed and appears to be a record of late cretaceous tertiary tectonic activity

alleghanian episode of k bentonite illitization in the southern appalachian basin mixed layer illite smectite i s from middle ordovician potassium k bentonites was found to be uniformly illitic throughout the southern appalachian basin regardless of variable depths of burial of the k bentonites the k ar ages of illitization are narrowly confined between 272 and 303 ma late pennsylvanian to early permian this suggests that illitization was a short lived episode coincident with and prompted by the alleghanian orogeny the illitization is explainable by the fluid expulsion hypothesis recently proposed by others e g oliver hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny these fluids are thought to be effective agents of illitization some of our k bentonite samples are stratigraphically and geographically close to mississippi valley type lead zinc deposits suggesting a similar mode of origin rapid illitization of shales in a fashion similar to that observed for the bentonites should have led to high pore water pressures that enabled them to act as ideal decollements during alleghanian thin skinned deformation

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

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

paleomagnetism of the cambrian moore hollow group texas evidence for a primary magnetization carried by detrital magnetite petrographic and paleomagnetic studies of the cambrian moore hollow group texas establish the presence of three magnetic minerals and several components of magnetization hematite is the dominant carrier of the magnetic remanence in most samples and is primarily a diagenetic replacement of preexisting fe bearing phases hematite dominant samples have characteristic directions which range from shallow easterly to shallow southerly this distribution suggests multiple components of magnetization acquired at various times in the paleozoic the paleopole obtained for an unmineralized breccia 135 7 e 15 4 n represents either a chemical remanent magnetization of middle ordovician to early silurian age or a vector addition of two or more cambrian through silurian components mineralized breccias at the silver creek mississippi valley type deposit yield a permian direction 117 0 e 51 6 n because hematite in this deposit predates mineralization the permian age represents a maximum age for the mineralization at silver creek many samples contain goethite as a replacement of fe dolomite glauconite and pyrite the direction of magnetization carried by goethite is near the present geomagnetic field or in a minority of samples antipodal to it rounded ti magnetite was identified by sem in magnetic extracts from siliciclastic poor facies irm acquisition and thermal demagnetization confirms the presence of magnetite in these samples some samples contain an apparent modern viscous remanent magnetization the dominant component 72 samples 10 sites has a site mean declination of 93 8 and inclination of 14 5 with an 95 8 5 corresponding to a pole position at 163 0 e 0 6 s the presence of detrital magnetite polarity changes lack of evidence for prolonged burial and the proximity of this pole position to other late precambrian cambrian poles suggest that this component of magnetization indeed reflects the late cambrian paleopole for cratonic north america

absolute dating of dedolomitization and the origin of magnetization in the cambrian morgan creek limestone central texas goethite associated with dedolomite in the morgan creek limestone in central texas carries a chemical remanent magnetization crm the pole position 88 n 96 w k 67 95 2 7 which is close to that of the modern pole and the fact that only normal directions are present suggest that the component is a post brunhes 700 000 yr crm dedolomite is common in the morgan creek and consists of calcite rhombohedra with relict dolomite goethite occurs surrounding the rhombs and in rhombic zones within the crystals the goethite precipitated when iron was released from ferroan dolomite during the dedolomitization process the paleomagnetic data therefore date the dedolomitization process as modern dedolomitization is probably related to surface weathering of the morgan creek in addition to goethite associated with dedolomite the morgan creek contains hematite replacing fossils and glauconite and detrital hematite the morgan creek also contains three other magnetic components one of these components is a viscous remanent magnetization aligned with the present magnetic field another component is a triassic jurassic crm residing in hematite a fourth component in the morgan creek is an early paleozoic magnetization residing in hematite and minor magnetite and represents the vector addition of a detrital remanent magnetization and one or more crms the results of this study demonstrate that paleomagnetic and petrographic techniques can be used to date a diagenetic event and also illustrate the complex nature of magnetization in a sedimentary unit

hydrothermal alteration in anthracite from eastern pennsylvania implications for mechanisms of anthracite formation orthogonal joint sets cleat in anthracite rank coal beds from eastern pennsylvania contain two mineralogically and chemically different authigenic clay mineral assemblages in localities from all four anthracite fields the systematic cleat contains significant quantities 20 to 95 wt of authigenic rectorite sudoite or tosudite which are clay minerals primarily associated with hydrothermal veins and ore deposits the strike of systematic cleat in this region roughly parallels the inferred direction of lateral compressive stress during the alleghany orogeny the opposing nonsystematic cleat contains mostly authigenic nh 4 illite a clay that also occurs in the coal matrix all of these authigenic clay minerals formed during anthracitization t 200 c by replacement of kaolinite and quartz both of which formed during an earlier stage of coalification and are present in the joints and matrix of nearly all coal samples these distinct mineralogical structural relations are observed throughout the anthracite region and suggest that 1 mineralogical variations in the coal joints are related primarily to permeability variations that were controlled by the orientation of alleghanian stress fields 2 hydrothermal fluids passed through the systematic cleat during anthracitization and 3 hydrothermal alteration influenced diagenesis in this region hydrothermal alteration may be related to basin wide fluid migrations that were driven by alleghanian age uplift basinal fluid flow concentrated along permeable joints and detachment zones could have efficiently transported heat from depth and thereby increased the rate of coalification and decreased the postulated minimum depths at which anthracitization occurred perhaps 5km vs 6 10 km

scanning and transmission electron microscope observations of magnetite and other iron phases in ordovician carbonates from east tennessee previous paleomagnetic observations for the carbonates of the lower ordovician knox group have indicated that ancient magnetizations in these rocks are of the same age as the late paleozoic alleghenian orogeny rock magnetic properties strongly suggest magnetite as the carrier of the magnetization but the textural and crystalline characteristics sizes morphologies and mineral associations of these magnetites are poorly known we have examined magnetic extracts and iron oxides in thin sections with scanning sem and scanning transmission stem electron microscope techniques to determine whether the observed iron oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers several different forms of magnetite in limestones and dolomites which in places are host to mississippi valley type deposits are documented by imaging and energy dispersive analysis using sem and stem by x ray diffraction and electron diffraction patterns using stem the magnetite is either spherical with a dimpled surface or nonspherical in the form of void filling single grains or grain aggregates most of the iron oxides have the composition of pure end member magnetite but occasional titanomagnetite and hematite including rare zincian hematite have been observed only in limestone wherever found in thin section nonspherical magnetites occur in association with secondary dolomite potassium feldspar illite and quartz some iron oxides have in fact inclusions of k feldspar and quartz some of the magnetite spherical and nonspherical is polycrystalline this implies that the larger observed grains may consist of single domains or pseudo single domains this provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single domain but interacting subgrains on the basis of remanent coercivities and blocking temperatures we interpret the pure end member magnetite to be authigenic having formed at approximately the same time as the k feldspars which in nearby areas have yielded late paleozoic radioisotopic ages 278 322 ma the knox carbonates therefore are inferred to carry a chemical remanent magnetization iron rich clays or original iron titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution precipitation processes these processes require rock fluid interactions which are thought to be related to migrating connate brines during the alleghenian orogeny

paleomagnetism of the methow region north central washington structural application of paleomagnetic data in a complexly deformed variably remagnetized terrane jurassic through paleocene rocks of the methow pasayten belt were studied in order to use paleomagnetic directions to resolve the question of cretaceous northward transport in the end circumstanc

combined numerical and geomorphological reconstruction of the serra da estrela plateau icefield portugal abstract the paper focuses on reconstructing a plateau icefield surface from field geomorphological data and a physical based glacier model the results allow the analysis of the patterns of glacial erosion and the estimation of the palaeo equilibrium line altitudes ela the study area is the serra da estrela a plateau in central portugal rising to 2000 m asl the glaciated area during the last maximum of the serra da estrela glaciation lmgse was 66 km 2 the reconstruction of the topography of the icefield and valley glaciers in the lmgse is based on the schilling and hollin model it iterates along the valley longitudinal profile and is based on the gradient on valley shape indices and on yield basal shear stresses these variables influence ice thickness and therefore also the slope of the ice surface allowing for its reconstruction the key variable is basal yield shear stress and its value was included in the model manually starting from the points of maximum extent of the valley glaciers and following a range known to occur in contemporary conditions the input values are validated by matching the resulting ice surface to geomorphological features where these are absent a constant value of 100 kpa was used the icefield was reconstructed from a radiating set of long sections along which ice thicknesses were calculated a dem of the ice surface was constructed allowing the estimation of the hypsometric curves of the distinct glacier catchments and the calculation of the palaeo elas the results show a regional ela at 1650 m asl with spatial variations across the icefield reflecting mainly the effect of eastward snow drift the lmgse glaciers were very sensitive to minor climatic changes especially due to the large area of the plateau icefield and to the positioning of the elas close to or in the flat part of the hypsometric curve the model of the ice surface is of significant value for the analysis of the patterns of glacial erosion at the landscape level in the serra da estrela most of the glacial erosion occurred near the plateau margins and in valley heads where glacier surface slope was steeper allowing for a faster ice flow and where ice flow concentrated strong glacier erosion in the zezere valley is linked to the tectonic setting but also to the confluence of glaciers and to the overfeeding of snow from the plateau

paleomagnetism of the mississippi valley type ores and host rocks in the northern arkansas and tri state districts a paleomagnetic study has been done on 452 specimens from the early ordovician cotter and middle ordovician everton formations in the northern arkansas mississippi valley type mvt lead zinc ore d

palaeomagnetism of the early carboniferous deer lake group western newfoundland no evidence for mid carboniferous displacement of acadia early carboniferous visean to possibly earliest namurian sedimentary rocks of the deer lake group of western newfoundland rest unconformably on grenvillian basement rocks of the canadian shield which form the western border of the early palaeozoic appalachian orogen in addition to magnetically soft magnetizations directed along the present field three families of magnetization directions are found two of them referred to as n north and s south are antiparallel and prefolding and were probably acquired during the depositional process n and s are roughly of equal frequency they have a mean direction irrespective of sign of 0 7 35 2 k 40 95 8 9 and a palaeopole 21 5 n 121 8 e 10 3 6 0 corresponding to a palaeolatitude of 20 6 s this agrees with the palaeolatitude 17 5 s determined from the somewhat older early carboniferous tournaisian terenceville formation of the avalon platform on the eastern side of the appalachian orogen in newfoundland the third magnetization referred to as h roughly horizontal has a mean direction 156 8 13 3 k 37 95 10 1 and a palaeopole 45 4 n 140 3 e 10 3 5 3 corresponding to a palaeolatitude of 7 4 n we interpret this to be an early kiaman latest carboniferous to early permian overprint probably acquired chemically the palaeolatitude determined from the h overprint agrees with that determined from early carboniferous rocks of cratonic north america west of the appalachians therefore we argue early carboniferous palaeofield determinations for cratonic north america have been strongly biased by unremoved kiaman overprints because of this and because of the good agreement between early carboniferous palaeolatitudes obtained from opposite margins of the appalachian orogen we suggest that there is at present no palaeomagnetic evidence for the previously proposed 1500 km displacement from the south of an eastern portion of the appalachians acadia relative to cratonic north america during the carboniferous

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin