magnetostratigraphy of the lebo and tongue river members of the fort union formation paleocene in the northeastern powder river basin montana we analyzed paleomagnetic samples and documented the stratigraphy from two sections near miles city montana to determine the geomagnetic polarity stratigraphy and to constrain the age and duration of the lebo and tongue river members of the fort union formation in the northeastern powder river basin the resulting polarity sequence can be correlated to subchrons c29n c26r of the geomag netic polarity time scale by interpolating measured sediment accumulation rates from the base of c28r to the top of c27n and then extrapolating to the top of the tongue river member and the bottom of the lebo member we developed two age models to estimate the durations of the lebo and tongue river members based on the first model which uses different sedimentation rates for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 30 and 1 74 million years and of the tongue river to be between 1 42 and 1 61 million years using the second model which uses the same sedimentation rate for the lebo and tongue river members we estimate the duration of deposition of the lebo to be between 1 33 and 1 76 million years and of the tongue river to be between 1 00 and 1 25 million years our results indicate a decrease in sediment accumulation rates in c27r which is likely the result of a 0 26 to 0 62 million year long depositional hiatus in the middle of c27r represented by the lebo tongue river contact this unconfor mity occurs 2 million years earlier than previously suggested and is likely contempo raneous with unconformities in the williston basin and in southwestern alberta suggesting that it may be regionally significant

a theoretical treatment of paleointensity determination experiments on rocks containing pseudo single or multi domain magnetic particles a phenomenological theory of thermoremanent magnetization trm is applied to model the possible results of paleointensity determination experiments using the thellier method it is shown that intrinsic curvature of the arai plot for samples containing pseudo single or multi domain particles can be attributed exclusively to the presence of remanences which are unblocked below their blocking temperature thus the thermal demagnetization tail of partial trm ptrm is not a direct measure for a sample s tendency to yield a curved arai plot here it is proposed to use the advancing decay of a ptrm t1 t2 which during heating occurs already below the lower temperature t1 t2 as a more reliable pre selection criterion in addition the effect of an average magnetostatic interaction on the thellier experiment turns out to be negligible an extended thellier method which compares the thermal demagnetization of natural remanent magnetization with that of an artificial trm according to the applied trm theory should result in a perfectly linear modified arai plot even for pseudo single or multi domain samples if no alteration or viscous effects occur

a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities 1 an extensive paleointensity study was carried out on an approximately 14 1 myr old reverse to normal transition of the geomagnetic field a total of 188 samples from a volcanic sequence on gran canaria canary islands were subjected to thellier type paleointensity determinations the paleodirectional results of this sequence are reported by leonhardt et al 2002 a modified thellier technique which facilitates the recognition of md tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step was used on the majority of the samples we obtained reliable paleointensity results for 35 of the 87 sampled lava flows in general the obtained field intensities are lower than expected for the mid miocene this observation is very likely related to a long term reduction of the field close to transitions very low paleointensities with values 5 t were obtained during an excursion preceding the actual transition and also close to significant changes of the local field directions these are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions during the transition 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to south america chronologically within this cluster the paleointensity increases from about 9 to 28 t followed by a decrease back to approximately 9 t this variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period

upper cretaceous paleocene biostratigraphy and magnetostratigraphy hell creek and tullock formations northeastern montana fossils from the hell creek and tullock formations in northeastern montana provide detailed documentation of terrestrial faunal and floral evolution during latest cretaceous lancian and early paleocene puercan time here the replacement of lancian faunas by those of puercan age most obviously signaled by the extinction of dinosaurs and the changes in pollen floras sometimes used to mark the cretaceous tertiary boundary occurred during a period of reversed magnetic polarity paleontological correlations suggest that dinosaur extinction and the change in pollen floras took place in the red deer valley area alberta during the same period of reversed polarity furthermore also on the basis of paleontological correlations the extinction of dinosaurs in the san juan basin new mexico appears to have occurred either during the same period of reversed polarity or possibly during the preceding period of normal polarity

magnetostratigraphy of the ludlow member of the fort union formation lower paleocene in the williston basin north dakota to determine the geomagnetic polarity stratigraphy and the duration and age of the ludlow member of the fort union formation lower paleocene we constructed a 325 m composite lithostratigraphic section of the upper cretaceous hell creek formation and the lower paleocene ludlow and tongue river members of the fort union formation in the little missouri river valley of north dakota usa we analyzed paleomagnetic samples from nine of the logged sections the principal magnetic carrier in the ludlow member sediments is likely titanomaghemite as indicated by predominantly irreversible thermomagnetic curves measured from sandstone siltstone and carbonaceous shale samples the analyzed paleomagnetic samples document a series of polarity zones that can be correlated from c29n to c27r on the geomagnetic polarity time scale gpts we infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the paleocene of north america and the reversal stratigraphy from this section corresponds to the gpts with reasonable sediment accumulation rates by extrapolating the measured sediment accumulation rate from the cretaceous tertiary k t boundary to the top of c28n and then to the top of the ludlow member we estimate the duration of the member to range from 2 31 to 2 61 m y this is the fi rst estimate for the duration and age of the ludlow member and it can be used as an important tool for interpreting rates of biotic recovery after the k t extinction

late brunhes polarity excursions mono lake laschamp iceland basin and pringle falls recorded at odp site 919 irminger basin abstract component natural remanent magnetizations derived from u channel and 1 cm3 discrete samples from odp site 919 irminger basin indicate the existence of four intervals of negative inclinations in the upper brunhes chronozone according to the age model based on planktic oxygen isotope data these excursional intervals occur in sediments deposited during the following time intervals 32 34 ka 39 41 ka 180 188 ka and 205 225 ka these time intervals correspond to polarity excursions detected elsewhere known as mono lake laschamp iceland basin and pringle falls the isotope based age model is supported by the normalized remanence paleointensity record that can be correlated to other calibrated paleointensity records for the 0 500 ka interval such as that from odp site 983 for the intervals associated with the mono lake and laschamp excursions virtual geomagnetic poles vgps reach equatorial latitudes and mid southerly latitudes respectively for intervals associated with the iceland basin and pringle falls excursions repeated excursions of vgps to high southerly latitudes indicate rapid directional swings rather than a single short lived polarity reversal the directional instability associated with polarity excursions is not often recorded probably due to smoothing of the sedimentary record by the process of detrital remanence drm acquisition

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

evidence suggesting extremely rapid field variation during a geomagnetic reversal abstract large systematic variations in direction of high temperature remanence as a function of vertical position occur in a basalt flow from the miocene volcanic sequence at steens mountain oregon that has provided a detailed record of a geomagnetic reversal these may have been caused by an impulsive change in the transitional field as the flow cooled during the reversal if this is true the short time required for the flow to cool by conduction and acquire its thermoremanent magnetization implies astonishingly high rates of change of the geomagnetic field at least 3 and 300 gammas per day

role of the olistostromes and olistoliths in tectonostratigraphic evolution of the silesian basin in the outer west carpathians abstract the silesian series deposited in the proto silesian and silesian basins and preserved within the silesian nappe represents most complete continuous basinal sedimentary succession late jurassic early miocene within the outer carpathians it contains intercalations of numerous olistostromes that occurred during every stage of flysch basin evolution from rift and post rift through orogenic to postorogenic stage the olistostromes represent sediments of sub marine debris flows and avalanches they are of different size and composition and were derived from ridges and their slopes bordering proto silesian and silesian basins 0however the main source was the tectonically active southern slope and adjacent ridges the silesian and bukowiec ridges composition of olistostromes is an important source of knowledge about the geological structure and history of ridges and basin slopes which now are not preserved some large olistoliths and olistoplaques have been incorrectly interpreted as thrust sheets so detail studies of their true origin allowed correct interpretation of local tectonics

deep sea trace fossils of the oligocene miocene numidian formation northern tunisia abstract twenty two ichnogenera and thirty one ichnospecies have been recorded in the oligocene miocene numidian formation of northern tunisia heterolithic successions of thin bedded turbidite sandstones and interchannel mudstones contain the most diverse trace fossil assemblages thick to very thick bedded structureless sandstones and conglomerates representing the fill of channel complexes contain a low diversity trace fossil assemblage the ichnoassemblage in the lower part of the formation oligocene which includes paleodictyon isp scolicia strozzii spirorhaphe isp cosmorhaphe isp and halopoa isp can be ascribed to the paleodictyon ichnosubfacies of the nereites ichnofacies the ichnoassemblage in the upper part of the formation miocene aquitanian including diplocraterion cf habichi scolicia vertebralis and ophiomorpha isp is interpreted as the shallower part of the ophiomorpha rudis ichnosubfacies of the nereites ichnofacies the notable switch in ichnofauna between the oligocene and lower miocene reflects variation in environmental and depositional conditions the common occurrence of trace fossils of the ophiomorpha rudis ichnosubfacies and diplocraterion cf habichi in the early miocene indicates an increase in energy level greater environmental disturbance and probable shallowing this is also confirmed by a corresponding decrease in the abundance and diversity of benthic foraminifera the integration of ichnological sedimentological and microfossil contents has allowed the distinction of two quite distinct geographical depositional settings within the numidian formation the first domain includes the numidian succession of the tabarka cap negro cap serrat and bouhertma areas which are characterised by distal turbidites showing the paleodictyon ichnosubfacies that is compatible with a lower bathyal depth in the oligocene and an upper slope depositional environment during the early miocene the second domain includes the ne part of mogod mountain e g the ras el korane jebel gattous zoukar which exhibit more proximal characteristics compatible with a probable slope canyon interpretation the southern margin of the kroumirie balta and zouza areas and sejnene area shows a distal setting compared with the ras el korane and jebel gattous zoukar areas it is ascribed to a mid to upper slope depositional environment during the oligocene to early miocene

paleomagnetic constraints on the tectonic history of the foreland belt southern canadian cordillera preliminary results in the southern canadian cordillera the paleomagnetic memory of paleozoic carbonate strata in the front ranges and inner foothills of the foreland thrust and fold belt retains no record of their known deposition at low latitudes instead each folded structure exhibits a similar but asynchronous sequence of events including an eastwardly progressing predeformational chemical remagnetization during the cordilleran orogeny the remagnetization of a western front ranges structure occurs during a period of normal polarity before 130 ma the paleomagnetic pole requires that the subsequent deformation of the western front ranges is jurassic or younger the remagnetization of a front ranges structure in the lewis thrust sheet occurs during a period of normal polarity after 130 ma but before deformation which from other evidence occurred around 75 ma the predeformational remagnetization of an inner foothills structure occurs during a reversed magnetic period that we interpret to be after 75 ma an early cretaceous sill in the lewis thrust sheet was remagnetized during a reverse polarity chron prior to the end of lewis thrust deformation when about 70 of the present dip of the sill was acquired remagnetization consistently predates deformation whereas it occurs later at more easterly localities there are also similarities in character and style of the remagnetizations among localities when coupled with the eastward progression of the deformation our observations suggest that an important and pervasive but hitherto unrecognized and unappreciated orogenic chemical process affected paleozoic carbonate strata in the van of the deforming cordilleran tectonic wedge

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

paleoclimate estimates and fire ecology immediately prior to the end cretaceous mass extinction in the frenchman formation 66ma saskatchewan canada abstract the fossil rich deposits of the uppermost maastrichtian 66 ma frenchman formation of southern saskatchewan canada provide a detailed continuous record of terrestrial ecosystem dynamics during the last half million years leading up to the end cretaceous mass extinction paleoenvironmental interpretations for the frenchman formation are here refined by the first absolute paleoclimate estimates for the late cretaceous of saskatchewan and by the placement of these climate estimates within a north american context climate estimates were calculated using leaf margin analysis and the climate leaf analysis multivariate program clamp derived from two independent plant macrofossil assemblages collected from the frenchman formation climate parameter estimates indicate that the maastrichtian climate of southern saskatchewan was subtropical to warm temperate with mean annual temperatures between 12 c and 14 c and with marked seasonality these results are comparable with estimates derived from other late campanian and maastrichtian paleofloras in the western interior basin although small variations among coeval localities were noted or determined overall the plant fossil assemblages from the two study localities support the paleoenvironmental interpretation of the frenchman formation as a swampy to lowland forest system with canopy forming deciduous conifers and a diverse angiosperm dominated mid canopy and understory an unexpected marked disparity in plant diversity was found between the two paleofloras despite the plant assemblages returning similar paleoclimate parameter estimates abundant in situ inertinite fossil charcoal deposits were found in association with the less diverse paleofloral site which contained only angiosperm fossils inertinite was absent from the deposits containing the more diverse paleoflora which contained a different composition of angiosperm and gymnosperm fossils fire is known to be a prevalent feature of late cretaceous deposits worldwide and modern angiosperm plants typically are the first to recover following fire disturbances we suggest that the two frenchman formation paleofloras represent different stages of secondary ecological succession following a disturbance by fire the identification of fire ecology in the fossil record may have important implications for understanding biodiversity patterns on small spatial and temporal scales immediately prior to the cretaceous mass extinction

did the european dinosaurs disappear before the k t event magnetostratigraphic evidence abstract debate on the non catastrophic or catastrophic extinction of the dinosaurs by an asteroid impact k t event remains a controversy and is mainly based on the interpretation of the sedimentary continental sequences in the north american western interior the global aspect of this event needs to be tested in sedimentary record from all continents where continuous terrestrial deposits through the cretaceous paleogene are well preserved in the western mediterranean realm recognition of the cretaceous paleogene boundary is limited by the lack of biostratigraphic data in the upper cretaceous lower tertiary continental sedimentary sequences new magnetostratigraphic results were obtained from the analysis of two sections in southern france and compared to previous results in northern spain the last occurrence of in situ dinosaurs eggshells the only dinosaur remains found is located in chron 30n southeast france or 31n southwest france and northern spain this last occurrence could demonstrate that the extinction of the european dinosaurs occurred prior to the cretaceous paleogene boundary and would therefore support the idea of a gradual or stepwise extinction unlinked to the k t event

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

macrofossil magnetobiostratigraphy for the upper santonian lower campanian interval in the western interior of north america comparisons with european stage boundaries and planktonic foraminiferal zonal boundaries magnetostratigraphic samples were collected from the milk river and pakowki formations of southeastern alberta to determine the polarity chron 34 33r boundary this boundary was located in the upper half of the deadhorse coulee member and is below the first local occurrence of the western interior ammonite baculites obtusus comparison with the laterally equivalent eagle sandstone of montana and niobrara formation of colorado indicates that the 34 33r contact is within either the baculites sp smooth or baculites sp weak flank ribs ammonite zone in the western interior of north america direct correlation of the desmoscaphites erdmanni through baculites asperifomis western interior ammonite zones with magnetobiostratigraphic sections of the upper santonian lower campanian stages in europe suggests that i the santonian campanian boundary as defined in europe is within the lower baculites obtusus zone and ii the western interior appearance of some ammonites and crinoids scaphites hippocrepis trachyscaphites spiniger marsupites uintacrinus is profoundly time transgressive relative to the appearance of these same species in europe the date of 84 ma for the santonian campanian boundary is too old and a date of around 80 ma may be more accurate the santonian campanian boundary defined by ammonites crinoids and belemnites in europe is within the lower part of chron 33r and is younger than the santonian campanian boundary as determined by planktonic foraminifera at gubbio italy

core flow instabilities and geomagnetic storms during reversals the steens mountain impulsive field variations revisited abstract the origin of the large variations in magnetization direction recorded in two different lava flows at steens mountain oregon is discussed the time constant for lava flow cooling and the amplitude of the variations imply extremely large variation rates for the ambient field four orders of magnitude above the present day secular variation two different mechanisms in the liquid outer core namely turbulence and inertial waves that may produce such variations are investigated energetic considerations indicate that the observed impulsive variations are unlikely to be of internal core origin on the other hand it is shown that during reversals when the dipolar field is reduced by a factor ten magnetic storm perturbations at the earth s surface are enhanced by a factor two with respect to their present amplitude it follows that intense magnetic storms can significantly contribute to ambient field changes during reversals the amplitude of the impulsive variations recorded at steens mountain is shown to be consistent with the effect of a strong magnetic storm occurring during cooling of a flow the direction of the variations is also shown to be consistent with this mechanism if the dipolar field contributes significantly to the internal field during the impulsive change

phosphate lu hf geochronology abstract igneous metamorphic and biogenic apatite contains lu and hf in proportions that make this mineral suitable for dating by the 176lu 176hf method we present a new method for separation of lu and hf from apatite that involves a single extraction column step for hf and a second exchange column step for lu this procedure allows rapid sample processing prior to analysis by mc icp ms results from the igneous gardiner skaergaard and khibina intrusions indicate that the closure temperature for the lu hf isotopic system in apatite is relatively high the lu hf isochron age of 60 ma for the skaergaard intrusion east greenland is older than the generally accepted emplacement age but does not appear to be a product of mixing a centimeter sized apatite from the otter lake area grenvillian province yields high and variable 176lu 177hf ratios showing that metamorphic apatite may in some cases allow for mono mineral dating the most important application of the method is undoubtedly its potential use for dating sedimentary formations although we find biogenic apatite in porous matrix highly susceptible to post depositional resetting of the lu hf isotopic system the preservation of a near depositional age for a shark tooth from the impermeable london clay and recent work on crystalline authigenic phosphorites earth planet sci lett 201 2002 203 show the lu hf system to be a promising tool for direct dating of sedimentary formations

on the possibility of recovering paleo diurnal magnetic variations in transitional lava flows i constraints from thermoremanence modelling for an experimental protocol abstract one of the tenets in paleomagnetism is that perturbations of the ground magnetic field due to magnetospheric or ionospheric current systems are too small to leave a detectable paleomagnetic signature in lava flows as suggested by recent work in paleomagnetosphere modelling however external field perturbations may be significantly enhanced during periods of transitional field behavior particularly when the dipole field axis is strongly tilted towards the equator which then leads to an extremely dynamic magnetosphere on the diurnal time scale even for quiet solar wind conditions we here demonstrate that thin rapidly cooled lava flows 50 cm thick with high magnetic blocking temperatures within 50 100 c below the curie temperature indeed have the potential to record such diurnal perturbations further an experimental protocol is suggested to paleomagnetically extract these perturbations our proof of concept is based on numerical modelling of thermoremanence trm acquisition and simulation of thermal demagnetization surfaces for discrete temperature steps in function of vertical position in the flow the trm direction recovered at a given thermal demagnetization step varies with vertical position in the flow and thereby reveals the wave form of the external field variation characteristically the vertical position of a captured signal changes systematically with unblocking temperature which reflects the oblique orientation of cooling isochrons along which the signals are blocked the signals have their largest amplitudes at the maximum unblocking temperatures but decay away at lower temperatures it is by these systmatic trends that external field perturbations if trapped can be paleomagnetically identified and distinguished from a secondary overprint the experimental procedure requires a sample spacing of 1 cm with 1 cm drill cores and small thermal demagnetization steps 15 c at elevated temperatures

absolute paleointensity and magnetomineralogical changes determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization ptrm are reflected by negative ptrm checks a deviation between the original ptrm and a repeated measurement after heating at a higher temperature because the deviations indicated by the ptrm checks can provide a direct estimate of the effects to the ptrms we suggest that they can be used to correct ptrms the correction method we present requires additional measurements in particular ptrm checks performed after each ptrm and specific diagrams to detect acquisition of chemical remanence performing multiple successive ptrm checks can also provide information on the blocking temperatures of the alteration product these experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step the corrections have been tested on recent and historic lava flows from several localities we have also compared results on samples from the same lava flows that were heated in air and in vacuum the mean paleointensity values were found to lie within less than 10 from the expected field intensity at each site the corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65 additional data for specimens heated in air successive heatings at the same temperature in air are often accompanied by a time dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field a problem that is not related to the corrections we conclude that corrections can be used with fairly high confidence to the ptrms performed in vacuum and in air

radiometric time scale for the upper eocene and oligocene based on k ar and rb sr dating of volcanic biotites from the pelagic sequence of gubbio italy several samples of unaltered volcanic biotite were recovered from five biostratigraphically and magnetostratigaphically controlled bentonite horizons in the continuous upper eocene oligocene pelagic carbonate sequence of gubbio northern italy k ar and rb sr dates on these samples offer a unique opportunity to calibrate with closely spaced points the late paleogene biostratigraphic and magnetostratigraphic time scales until now time scales have been constructed by interpolating between less certain and more widely spaced data points our combined k ar and rb sr dates are here used to calibrate the following five stratigraphic levels from top to bottom level 5 upper part of globorotalia opima opima planktonic foraminiferal zone upper part of indistinct np23 np24 nannoplankton zone and cp18 nannoplankton zone bottom of magnetic anomaly 9n 28 0 0 3 ma level 4 globigerina ampliapertura and cassigerinella chipolensis zone lower part of np23 zone and bottom of cp18 zone upper part of anomaly 12r 32 0 0 8 ma level 3 pseudohastigerina micra zone upper part of np21 and cp16b zones top of anomaly 13n 35 4 0 4 ma level 2 upper part of globigerinatheka semiinvoluta zone upper part of np18 and cp15a zones top of magnetic anomaly 16n 36 0 0 4 ma level 1 lower part of g semiinvoluta zone between zones np18 and np19 middle part of zone cp15a upper part of anomaly 17n 36 4 0 3 ma from these data the interpolated age for the much debated eocene oligocene boundary is 35 7 ma with a 2 standard deviation uncertainty of 0 4 ma

correlation of grassy lake and cedar lake ambers using infrared spectroscopy stable isotopes and palaeoentomology the late cretaceous grassy lake and cedar lake amber deposits of western canada are among north america s most famous amber producing localities although it has been suggested for over a century t

transitional geomagnetic impulse hypothesis geomagnetic fact or rock magnetic artifact a striking feature of the steens mountain oregon geomagnetic polarity reversal is the two maybe three extremely rapid field directional changes 6 degrees per day proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow each of these very fast field changes or impulses is associated with a large directional gap some 90 in the record in order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers we have carried out a paleomagnetic investigation of two new sections b and f in the steens summit region which cover the second and the third directional gap the main result is the description of two new directions which are located between the pre second and post second impulse directions these findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations which now appears to be more ad hoc as an explanation of the paleomagnetic data however the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section steens a and d 1 5 km to the north in addition new results for 22 transitional and normal lava flows in section b are presented that correlate well with earlier results from section a

the terminal cretaceous iridium anomaly in the red deer valley alberta canada neutron activation analysis following fire assay preconcentration in a nickel sulphide button allows processing of relatively large samples 25 g for precision analysis of platinum group elements using this method an iridium anomaly with maximum concentration of 3 36 0 04 ppb has been located at the cretaceous tertiary boundary at the base of the nevis coal seam in the red deer valley of alberta the iridium anomaly coincides with the disappearance of diagnostic late cretaceous palynomorphs including all but one species of aquilapollenites

marine transgressions and the evolution of cretaceous dinosaurs fluctuations transgressions and regressions of intercon tinental sea ways during the mesozoic era affected the habitat area of coastal plains by alternately restricting the space during transgressive phases and expanding the space during regressive phases in western north america a coastal plain stretched along the eastern front of the young rocky mountains during most of the late cretaceous here we report results of a six year field study of the sediments and dinosaur remains from this coastal plain in montana that provide evidence that anagenesis characterized dinosaur evolution at this time and that the evolutionary pulse coincided with a marine transgression

40ar 39ar age and correlation of the nonmarine two medicine formation upper cretaceous northwestern montana u s a the age of the nonmarine two medicine formation of northwestern montana is currently based upon correlations with k ar dated western interior ammonite zones 40ar 39ar dating of biotite and plagioclase separated from four bentonites and one crystal rich tuff permits for the first time direct determination of the age of two medicine strata biotite and plagioclase from a bentonite 10 m below the top of the two medicine formation yield concordant 40ar 39ar ages of 74 ma while biotite and plagioclase from two bentonites and a crystal rich tuff from approximately 100 m above the base of the formation cluster in age around 80 ma the total duration of two medicine deposition is estimated using these new radio isotopic ages via extrapolation of an average rock accumulation rate the new 40ar 39ar ages facilitate regional correlation of the dinosaur dominated paleofauna recovered from the two medicine formation and help constrain the timing of the claggett and bearpaw transgressions the ages support correlatio

field expressions of the transformation of debris flows into turbidity currents with examples from the polish carpathians and the french maritime alps various transformation mechanisms can generate turbidity currents from subaqueous debris flows different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits but the two approaches have not generally been linked this has made the genetic interpretation and comparison of deposits difficult in this paper a generic classification scheme of debrite turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments five different flow types called a e herein and their resulting deposits are detailed but they are all part of a continuous spectrum and a mixture of types is likely to be found in the field type a flows are strong dense debris flows that undergo little transformation their deposit will be a debrite overlain by a thin turbidite which is separated from it by a clear grain size break type b flows are weaker and can develop waves at the debris flow turbidity current interface the deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type a flows for type c flows the interfacial waves will grow so much that the debris flow disintegrates into separate parts the deposit will consist of debrite lenses encased in a turbidite type d flows will undergo even more mixing than type c flows so that the debrite parts will be mixed their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed type e flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration the deposit will be largely turbiditic the flow types and deposits will be illustrated with some examples from two field areas the polish carpathians and the french maritime alps

how the geomagnetic field vector reverses polarity a highly detailed record of both the direction and intensity of the earth s magnetic field as it reverses has been obtained from a miocene volcanic sequence the transitional field is low in intensity and is typically non axisymmetric geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur suggesting that liquid velocity within the earth s core increases during geomagnetic reversals

timescales and mechanisms of ree and hf uptake in fossil bones abstract rare earth element ree patterns of fossil bones and teeth are widely used as proxies for provenance taphonomy and palaeoenvironment in order to investigate if fossil bones behave as closed systems over geologic time ree profiles were analysed by la icpms along cross sections of 54 bones from various well characterised and well dated settings these include terrestrial and marine diagenetic environments covering early triassic to holocene ages in general all fossil bones exhibit the highest ree concentrations at the outer rim gradually decreasing by up to four orders of magnitude toward the inner bone cortex intra bone ree concentration gradients decrease significantly from quaternary via tertiary to mesozoic specimens suggesting long term ree uptake and open system behaviour of fossil bone this view is further corroborated by 176lu 176hf dating of selected samples all yielding significantly younger ages than the known chronostratigraphic ages hence there is clear evidence for long term open system behaviour of fossil bones with respect to ree which is in marked contrast to currently accepted models suggesting that ree uptake is only early diagenetic although unexpected statistically significant four to seven point isochrons are observed for four fossil dinosaur bone samples and one upper triassic mastodonsaurus tooth with mswds ranging from 0 083 to 4 5 notably mobility of lu alone cannot account for the observed age patterns assuming constant lu uptake rates over time the radiometric ages should only be as low as half of the chronostratigraphic age however a six point isochron defined by subsamples of a single upper triassic mastodonsaurus tooth yields an age of 65 2 1 1 ma mswd 0 68 much younger than half of the stratigraphic age ca 234 ma hence hf must also undergo late diagenetic exchange likely mechanisms to account for the presence of statistically meaningful isochrons as well as for the late diagenetic exchange of both ree and hf are diffusion adsorption and dissolution reprecipitation processes

abundant trace fossil polykampton in palaeogene deep sea flysch deposits of the lesser caucasus in georgia palaeoecological and palaeoenvironmental implications abstract the new ichnospecies of polykampton georgianum occurs abundantly in a part of the borjomi flysch upper paleocene lower eocene in georgia in particular in the 45 3 m thick ardagani 1 section that is dominated by grey silt and mudstones having sandstones intercalated the trace fossil occurs in the middle and upper part of the section the trace fossil is composed of a median tunnel and side lobes that are filled with material differing from the host sediment it is interpreted as a sequestrichnion wherein the tracemaker stored organic rich mud in the lobes as a food resource for times of food deficiency the today ferruginization of the median tunnel resulting from the former presence of pyrite suggests strong chemical gradient between its lumen and the surrounding chemichnial behaviour is not excluded as nutritional strategy of the worm like tracemaker a polychaete it occupies a middle tier and is commonly crosscut by trichichnus chondrites and scolicia rare or absent other trace fossils typical of deep sea fine grained deposits suggest slight dysoxia significant impoverishment of the tiering pattern only trichichnus present and absence of trace fossils in the lower part of the section point to strong dysoxia or episodic anoxia the trace fossil assemblage is atypical but in general it resembles the nereites ichnosubfacies of the nereites ichnofacies which characterizes distal parts of the turbiditic depositional systems the contrast between the dark grey to grey silt and mudstones deposited during the waning phase of gravitational flows and greenish pelagic mud suggests that the former originated from an organic rich depositional setting presumably within the oxygen minimum zone in the upper slope to outer shelf domain sequestration of the organic rich mud was a successful adaptation of the polykampton georgianum producers in this specific environmental setting

palynostratigraphic zonation of paleocene strata in the central and south central alberta plains paleocene strata of the scollard and paskapoo formations in the central and south central alberta plains can be subdivided into five formal biostratigraphic zones based in part on a momipites cary

a new centrosaurine ceratopsid from the oldman formation of alberta and its implications for centrosaurine taxonomy and systematics centrosaurus brinkmani sp nov is distinguished from centrosaurus apertus by key features of its cranial ornamentation including the shape and orientation of the postorbital horn and parietal ornamentation at parietal locus 3 the shape of the parietal ornamentation at locus 2 and the possession of accessory parietal ossifications developed as short spines on the caudal parietal ramus this species is restricted to the oldman formation of southern alberta and is the oldest ceratopsid represented by diagnostic material in canada phylogenetic analysis of the centrosaurinae suggests that the development of spike like ornamentation at the parietal locus 3 parietal locus is inversely related to the development of the p1 parietal ornamentation

bentonite chemical features as proxy of late cretaceous provenance changes a case study from the western interior basin of canada abstract bentonite beds are fairly common in both marine and terrestrial upper cretaceous campanian maastrichtian deposits of the western interior basin of western canada and northwestern united states a detailed stratigraphic sedimentologic geochemical x ray fluorescence and mineralogical x ray diffraction study of twenty one bentonites from the puskwaskau and wapiti formations in the grande prairie area west central alberta canada is here presented major and trace element concentrations from altered volcanic ashes document the presence in the study area of predominantly trachyandesitic and rhyolitic volcanogenic products resulted from intense volcanic arc to within plate pyroclastic activity concentration values of high field strength elements hfse and selected large ion lithophile elements lile e g nb zr th and y obtained by x ray fluorescence spectroscopy strongly support the presence of multiple volcanic sources integrated paleoenvironmental and geochemical criteria for provenance determination indicate a bimodal occurrence of basic and acid volcanic products interpreted as reflection of source areas characterized by different tectonic setting and magmatic composition a comparative analysis of geochemical compositions between grande prairie bentonites and 30 known volcanic beds from central and southern alberta manitoba and montana 1 documents a trend toward more acidic and alkali depleted volcanic products during the late campanian early maastrichtian interval and 2 suggests a well constrained stratigraphic and geographic subdivision of the non marine successions of the foreland basin on the basis of geochemical characteristic of volcanic ash beds furthermore geochemical fingerprints of several decimeter to meter thick bentonite beds have been coupled with volcanic ash subsurface signature in order to investigate their role as marker beds this multiple approach provides a reliable tool for basin scale identification and correlation of non marine sedimentary successions

absolute paleointensities and paleodirections of miocene and pliocene lavas from fernando de noronha brazil abstract forty four volcanic units were sampled on the equatorial atlantic island of fernando de noronha brazil twenty seven sites are from the 1 8 3 3 myr old quixaba formation and the others are from the approximately 8 12 myr old remedios formation analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles vgps and vgp dispersion s f of vgp lat 86 9 vgp long 136 3 s f 11 2 for the quixaba formation and vgp lat 86 8 vgp long 196 7 s f 15 5 for the remedios formation successful paleointensity determinations were obtained for 9 of the 27 sampled units in the quixaba formation a modified thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations ptrm and magnetization stabilization processes furthermore the additivity of ptrms was monitored during this modified technique of the thellier experiment these additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment such correction methods were used for analyzing approximately 50 of the determinations and lead to a reduction of the within site scatter the mean paleointensity result of 29 3 t corresponding to a virtual axial dipole moment vadm of 7 5 10 22 a m 2 yields approximately the present day field strength mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed today s geomagnetic field in the equatorial atlantic shows a significant dipole anomaly our results indicate that this anomaly was far less pronounced in the geological past

phanerozoic history of deep sea trace fossils the phanerozoic diversity of deep sea trace fossils based on 151 flysch formations displays distinct non linear changes through the phanerozoic with peaks in the ordovician early silurian and early carboniferous lowered in the permian older late jurassic a peak in the tithonian aptian lowered in the albian and the maximum in the eocene the contribution of graphoglyptids in trace fossil assemblages rises gradually up to the end of the cretaceous shows a peak in the palaeocene eocene but a depression in the oligocene all the changes were probably influenced by competition for food and food supply bottom water temperatures and oxygenation and frequency of flysch habitats there is no clear influence of major biotic crises ordovician silurian cretaceous tertiary palaeocene eocene on the diversity of deep trace fossils except the eocene oligocene crisis

reconstruction of cretaceous rifts incorporated in the outer west carpathian wedge by balancing abstract two balanced cross sections were constructed through the outer carpathians of poland in order to restore the early cretaceous rifts the rift fill is incorporated into the tertiary carpathian accretionary wedge the data compiled for this study confirm that the early cretaceous rifting in the silesian basin area was followed by late cretaceous paleocene basin inversion eocene pelagic deposition and oligocene syn orogenic deposition the original width of the silesian basin is about 130 138 km the overall wedge shortening ranges between 31 and 58 with 57 and 58 in the silesian nappe the rifting commenced at the jurassic cretaceous boundary and formed the horst and graben structures defined by nw se striking normal faults portions of the horsts become emergent at various times during rifting stages sedimentation rates in the grabens varied around 4 7 2 1 and 1 3 cm ka during three rifting stages while sedimentation rates on their slopes varied around 0 0 1 26 and 0 cm ka the altitude difference between horst tops and graben floors did not exceed 2 km the reconstructed widths of the horsts range between 17 5 and 18 3 km the reconstructed widths of several grabens in both cross sections have values 14 5 45 7 57 7 and 79 3 km the rifting related extension calculated from fault striae data was ne sw directed sometimes the very low 2 3 1 3 stress ratio of the driving stress configuration resulted in polydirectional extension when the value decreased below 0 1

deep sea sediment records of the laschamp geomagnetic field excursion 41 000 calendar years before present 1 we have recovered two new high resolution paleomagnetic records of the laschamp excursion 41 000 calendar years b p from deep sea sediments of the western north atlantic ocean the records document that the laschamp excursion was characterized locally by 1 declination changes of 120 2 inclination changes of more than 140 3 1200 year oscillations in both inclination and declination 4 near 90 out of phase relationships between inclinations and declinations that produced two clockwise loops in directions and virtual geomagnetic poles vgps followed by a counterclockwise loop 5 excursional vgps during both intervals of clockwise looping 6 magnetic field intensities less than 10 of normal that persisted for almost 2000 years 7 marked similarity in excursional directions over 5000 km spatial scale length and 8 secular variation rates comparable to historic field behavior but persisting in sign for hundreds of years all of these features with the exception of anomalously large directional amplitude are consistent with normal magnetic field secular variation comparison of our laschamp excursion paleomagnetic records with other late quaternary excursion records suggests that there is a group of excursions which we term class i which have strikingly similar patterns of field behavior and likely share a common cause as part of the overall core dynamo process three general models of secular variation are described that can qualitatively produce class i excursions on the basis of these observations we conclude that class i excursions epitomized by the laschamp excursion are more closely related to normal secular variation and are not necessarily a prelude to magnetic field reversal

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

on the possibility of recovering palaeo diurnal magnetic variations in transitional lava flows 2 an experimental case study abstract geomagnetic field variations of external origin may be enhanced during periods of transitional field behaviour particularly when the dipole moment is low in which case they are likely to leave a paleomagnetic signature in rapidly cooled lava flows to test this proposition we have resampled en bloc and studied in fine detail a thin transitional aa flow from a mid miocene lava sequence on gran canaria which was paleomagnetically investigated previously leonhardt r soffel h c 2002 a reversal of the earth s magnetic field recorded in mid miocene lava flows of gran canaria paleointensities journal of geophysical research 107 2299 doi 10 1029 2001jb000949 the flow is characterised by high unblocking temperatures an equatorial vgp position and a very low absolute palaeointensity of 2 t two slabs were cut out of the flow and sampled at 1 cm intervals along four vertical profiles running parallel to each other thermal demagnetisation was performed on two profiles using heating steps as small as 15 c at elevated temperatures the high temperature part of the unblocking spectrum was found to be remarkably constant across the flow as was the curie temperature of 540 c and the negligible anisotropy of magnetic susceptibility the exsolution lamallae observed under the microscope point to deuteric high temperature oxidation having occurred prior to the acquisition of the primary thermoremanent magnetisation while the absolute palaeointensity values vary only little with vertical position the magnetisation directions recovered by thermal demagnetisation vary considerably on average by some 20 at 500 c these large variations can be attributed to an overprint by secondary minerals formed by fluid diffusion around vesicles and low temperature oxidation since the secondary magnetisation recorded transitional directions as well the overprint must have occurred soon after emplacement the directional variations typically decrease in amplitude with increasing blocking temperature which is contrary to what would be expected if pronounced diurnal external field variations were trapped in the flow

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

new chronology for the early miocene mammalian faunas of kisingiri western kenya new total fusion k ar ages indicate that all of the fossiliferous formations that make up the lower part of the early miocene kisingiri sequence in western kenya at rusinga island mfwangano island and karungu were deposited during an interval of less than 0 5 million years at c 17 8 ma ago this contrasts markedly with k ar ages previously published from these detrital tuffaceous formations which suggested that they were deposited over an interval of as much as 7 million years between 23 and 16 ma overlapping the age ranges of all other east african early miocene sites including koru songhor napak bukwa loperot muruarot and buluk in addition the analytical problems revealed by the new kisingiri results cast doubt on biotite ages which provide dating for the most important sites thus the strong differences between the kisingiri fauna and those of koru sonhor and napak long held to be due to ecology because of the apparent overlap in ages may actually be due to a difference in time if this view of the geochronology is correct it may now be possible to identify adaptive trends and evolutionary succession in the east african early miocene faunas

the trace fossil gyrophyllites in deep sea siliciclastic deposits of the istebna formation upper cretaceous palaeocene of the carpathians an example of biologically controlled distribution abstract gyrophyllites kwassizensis glocker occurs abundantly in deep sea density current siliciclastics of the istebna formation silesian nappe outer western carpathians poland slovakia and czech republic close to the cretaceous palaeogene boundary it is composed of petal shaped lobes radiating from a thin shaft forming rosettes on a few levels in the sediment the trace fossil is interpreted as a feeding structure produced by a soft body worm which actively filled the lobes with mud transported from the underlying layer probably in response to migration of redox boundary in a generally poorly oxygenated environment probably due to microbial decomposition of cellulose and other organic particles the mud from below was more nutritional than the more fresh mud from the sea floor the number of lobes in a rosette from 7 to 15 is referred to ontogenetic development of the tracemaker and indirectly to time of burrow occupation the number varies between localities and may indicate a more hospitable habitat in the areas where the number is higher g kwassizensis occurs only in a narrow stratigraphic interval after a significant drop of sedimentation rate caused by non eustatic reasons in facies conditions which are not unique during the long history late jurassic miocene of the external carpathian subbasins this suggests that facies control was less important for distribution of this trace fossil than some undetermined biological factors which enabled successful but incidental invasion of the tracemaker to deep sea floor of one of the subbasins the silesian basin

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

was the 1970 geomagnetic jerk of internal or external origin annual mean data from worldwide magnetic observatories show that there was a jerk1 step change in the second time derivative in the geomagnetic field which took place over an interval of 2 yr around 1970 if such a short lived phenomenon originated in the earth s core and were still detectable after passing through the mantle to the surface this would imply a much lower conductivity for the lower mantle than had been believed previously here we show by an objective test that most of the jerk has an internal origin

sequential colonization of muddy turbidites in the eocene beloveža formation carpathians poland abstract a 100 350 m thick succession of centimeter thick fine grained turbidites form the lower mud dominated part of a prograding deep sea fan in the eocene belove a formation in the carpathian mountains of poland these strata contain over 50 ichnotaxa only six of which are common phycosiphon nereites chondrites scolicia halopoa ophiomorpha polished slabs and examination of cross cutting relationships show that post event colonization was sequential using modern analogues it can be shown that the colonization of different depth levels in the sediment at different times reflects changing geochemical conditions especially the re adjustment and re establishment of the redox boundary modern analogues allow an assessment of the timing of each colonization phase the phycosiphon producers entered first and occupied the well oxygenated muds 5 40 mm below the sediment surface as the redox boundary reformed nereites producers entered and probably utilized microbial matter just above the redox boundary about 15 30 mm below the sediment surface subsequently chondrites producers penetrated down to the oxygen deficient layers of the event bed or even deeper levels the halopoa producers entered with phycosiphon but reworked coarser layers contemporaneously with this sequential colonization deep permanent bioturbators which were unaffected by the event cross cut the post depositional suite the variability of the resultant ichnofabrics can be attributed in part to a patchy distribution of the benthos and the relative frequency of turbidite events five ichnofabrics which reflect the successive colonization episodes are recognized each was modified by elements of the permanent infauna

paleosol barometer indicates extreme fluctuations in atmospheric co2 across the cretaceous tertiary boundary we present an atmospheric p co 2 p is partial pressure curve showing extreme fluctuations for the interval between ca 77 and 63 ma in southern alberta canada using a paleosol barometer paleosol carbonate nodules micrite were collected from 40 bk horizons among 6 stratigraphic sections for stable carbon isotope analysis based on results from the study area declining atmospheric p co 2 from 1200 ppmv v is volume in the campanian to 780 ppmv in the maastrichtian correlates with late cretaceous climate cooling and falling sea level as documented in global records the remarkable rise in atmospheric p co 2 near 65 5 ma 1440 ppmv correlates with volcanic activity associated with the deccan traps rising sea level and warmer global climates the decline in atmospheric p co 2 760 ppmv at the cretaceous tertiary boundary and subsequent sharp rise into the danian 1000 ppmv occurred during static terrestrial temperatures and sea level this work provides compelling evidence that atmospheric p co 2 curves modeled for the phanerozoic do not offer the resolution needed to understand environmental conditions during catastrophic events in earth9s history

the first radiometric ages from the judith river formation upper cretaceous hill county montana a new isotopic age of ca 78 ma was obtained by the conventional k ar method and by laser fusion 40ar 39ar analysis of biotite and sanidine crystals from two bentonites in the judith river formation of kennedy coulee hill county montana the 40ar 39ar analyses yielded weighted mean ages of 78 2 0 2 ma biotite and 78 2 0 2 ma sanidine for sample 84mg8 3 4 and 79 5 0 2 ma biotite and 78 5 0 2 ma sanidine for sample 85mg7 16 1 these are the first isotopic age determinations from the judith river formation the dated bentonites in kennedy coulee bracket two significant judithian age mammal localities ucmp v77083 and v81234 the radioisotopic age of ca 78 ma for the hill county local faunas and correlations of other judithian local faunas with marine units suggest that the duration of the judithian was at least 5 ma judithian local faunas are probably correlative with the late campanian of the western interior similarities in the principal judithian mammalian faunas from alberta monta

revisiting the initial sites of geomagnetic field impulses during the steens mountain polarity reversal summary we present a new palaeomagnetic investigation of the two sites from the steens mountain oregon volcanic record of a miocene polarity reversal which were supposed to record very fast changes of the geomagnetic field or impulses mankinen et al 1985 prevot et al 1985a b approximately 130 cores were first drilled from the two initial sites belonging to sections a and b in order to obtain at least one detailed and complete vertical sampling of each lava flow thermal analyses of natural remanent magnetization complemented by some alternating field treatments low field thermo magnetic curves microscopic observations and electron probe analyses of key magnetic phases lead us to somewhat different conclusions for the first and the second impulses at site b first impulse we find that the dependence of the remanence direction on the sample vertical position in flow b51 does not imply a directional field change during flow cooling but is better explained by a thermochemical overprinting due to the overlying b50 flow however this conclusion does not challenge the existence of the first impulse because this field change seems to be recorded some 25 m away in flow b51 coe prevot 1989 at a place where it is thick enough for this record not to have been erased by the baking due to b50 regarding the second impulse restudied at site a our new findings are more comprehensively explained by a change in the field direction during cooling of flow a41 2 than by some overprinting using a simple model of flow cooling the angular rate of change of the field is estimated to have been of the order of 2 3 or 250 350 nt per day during the impulse this figure is similar to that previously obtained from site d some 250 m away however the directional paths describing the field change are somewhat different at the two sites new investigations are planned to try to understand the origin of this discrepancy

geomagnetic field behavior during the iceland basin and laschamp geomagnetic excursions a simple transitional field geometry we present four new records of the iceland basin excursion ibe and five new records of the laschamp excursion le obtained from rapidly deposited marine sediments in the north atlantic ocean the nordic seas the gulf of mexico the south china sea and the southern indian ocean marked minima in relative paleointensity correspond with the paleomagnetic directional changes associated with all of the excursion records the virtual geomagnetic pole vgp paths of the four ibe records are all similar the vgps move southward over europe and africa reaching the southern hemisphere three reach antarctica and then move to more eastern longitudes before returning northward over australia and east asia describing a large counterclockwise loop the same vgp pattern is observed in other published records the vgp paths observed for the le are similar to those of the ibe however they loop clockwise instead of counterclockwise despite the different sense of looping the marked similarity among the paths for the two excursions suggests that a similar relatively simple geometry dominated the transitional field during both the ibe and the le similar dynamo mechanisms must therefore have been active in the earth s core for both excursions the duration of the excursions is estimated at 2 000 years which supports the suggestion that a difference exists between the mechanisms for excursions and reversals however the coincidence of the longitudinal bands for vgps associated with excursions compared to some reversal paths could also indicate an inherent link between the mechanisms for reversals and excursions

paleointensity variations across the last geomagnetic reversal at la palma canary islands spain a detailed volcanic record of absolute paleointensity has been obtained for the period encompassing the last geomagnetic reversal from a succession of 69 distinct lava flows in two adjacent sections on the island of la palma the correlation between the two sections relies on the positions of the brunhes matuyama boundary and on the flow chronology obtained by a refined potassium argon k ar dating technique several pretransitional flows exhibit very shallow paleomagetic inclinations but they cannot be considered as really transitional paleointensity experiments conducted on 288 specimens provided 168 successful determinations 14 were obtained after correction for 50 lava flows that were considered to represent 45 distinct units the evolution of the paleofield intensity can be constructed by assuming uniform extrusion rates between three dated lava flows the period preceding the transition is characterized by large fluctuations with typical peak to peak amplitudes between 20 and 40 t which decrease down to 7 t prior to the transition the 100 kyr long interval following the reversal is associated with field intensities as high as 60 t and depict a strong and rapid restoration of the dipole field the virtual dipole moments vdms have been compared with sedimentary vdms generated from the record of relative paleointensity from ocean drilling program site 851 to which have been added non dipole components similar to the present geomagnetic field a reasonable fit can be made between the two curves which implies variations in the eruption rates but not incompatible with the uncertainties in the k ar age determinations thus these results can support but do not fully demonstrate the existence of asymmerical saw toothed variations in field intensity

reciprocal stratigraphy of the campanian paleocene western interior of north america abstract the claggett bearpaw and cannonball successions campanian paleocene of the western interior foreland system were deposited during a series of regional second and third order transgressions and regressions locally modulated by fourth and lower order cycles the reciprocal flexural behaviour of the foreland lithosphere in response to orogenic cycles of loading and unloading results in out of phase sequences on opposite sides of the basin which defines the concept of reciprocal stratigraphies the proximal and distal regions of the basin are separated by a few kilometre wide hinge zone of proximal to distal facies change with the mid point taken as a stratigraphic hinge line which defines the boundary between coeval marine transgressive and regressive systems tracts or between nonmarine sequences and correlative sequence boundaries hinge lines have been mapped in alberta saskatchewan montana and wyoming for consecutive time slices during the campanian paleocene interval areally the hinge line traces a semielliptical pattern outlining the region of maximum foreland basin subsidence it migrated at 10 km ma to the north in response to late cretaceous paleocene orogenic dextral transpression the hinge line also migrated eastward during the campanian and maastrichtian in response to thrust sheet advance and westward during the paleocene in response to the redistribution of orogenic load associated with a transition from transpression to transtension possibly accompanied by a visco elastic deepening and narrowing of the foredeep

paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

late cretaceous to early tertiary magnetostratigraphy of a continental sequence red deer valley alberta canada the red deer valley exposes a conformable 350 m thick late campanian to early paleocene fluvial deltaic coal bearing sequence named the edmonton group systematic sampling and paleomagnetic analysis of the dominant lithologies mudstones sandstones and shales were carried out over five sections correlated by coal seams to provide some 500 sites stratigraphic levels primary detrital remanent magnetization directions after alternating field cleaning produce a polarity reversal pattern correlatable to sea floor magnetic anomalies 29 33 the campanian maastrichtian boundary based upon the first appearance of wodehouseia and the maastrichtian paleocene cretaceous tertiary boundary based upon an ir anomaly associated with dinosaur and cretaceous palynofloral extinctions correlate closely magnetostratigraphically with the marine foraminiferal section at gubbio italy radiometric dating of bentonites has indicated the ages of these boundaries to be approximately 71 and 64 ma respectively

new evidence for extraordinarily rapid change of the geomagnetic field during a reversal palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at steens mountain oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day the evidence is large systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow which are most simply explained by the hypothesis that the field was changing direction as the flow cooled