field and laboratory testing of st peter sandstone the purpose of this project was to evaluate mechanical properties of st peter sandstone by in situ testing and to compare the field data with laboratory results direct shear tests were conducted to evaluate the strength dilatancy behavior and thin section microscopy was used to help explain the significant friction angles associated with the material st peter sandstone is nearly cohesionless but it possesses a friction angle of 57 63 at low confinement the large angle of internal friction at failure may be due to locking of sand particles or to postdepositional quartz overgrowths tests on pulverized densely packed sand and loosely packed sand were conducted in the same manner as the intact specimens and yielded friction angles of about 42 and 34 pressuremeter tests were performed in situ and the results were interpreted using an elasto plastic analysis in terms of total stresses by appropriate consideration of system stiffness young s modulus was found to be about 0 5 gpa slightly lower than the laboratory value although unload reload cycles were not attempted assuming associative behavior the friction angle was estimated to be at least 56

geomagnetic paleosecular variation in the brunhes period from the island of el hierro canary islands we have sampled a sequence of 69 flows in the island of el hierro canary islands 46 of them belong to the el golfo formation and 23 to the most recent rift formation radiometric dating brackets this section between 442 ka and 134 ka with six intermediate datings all the samples were stepwise demagnetized partly with alternative field partly thermally with very similar results the large majority of the samples are characterized by a single stable component of magnetization chrm isolated after the first steps of demagnetization are all of normal polarity inclinations are on average shallower than expected from a geocentered dipole field but consistent with results from other areas at the same latitude mean declinations of the rift and el golfo sequences are both easterly deviated about 5 and 15 respectively and significantly different suggesting that the section has undergone a progressive clockwise rotation vgp scatter and directional scatter calculated for the rift sequence are consistent with available data from other geographical areas and also with recent models of paleosecular variation on the other hand a significantly smaller directional scatter is obtained for the lower el golfo sequence suggesting that a period of low secular variation occurred in the canary islands area between approximately 440 and 260 ka

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

formation of fold and thrust belts on venus by thick skinned deformation on venus fold and thrust belts which accommodate large scale horizontal crustal convergence are often located at the margins of kilometre high plateaux1 5 such mountain belts typically hundreds of kilometres long and tens to hundreds of kilometres wide surround the lakshmi planum plateau in the ishtar terra highland fig 1 in explaining the origin of fold and thrust belts it is important to understand the relative importance of thick skinned deformation of the whole lithosphere and thin skinned large scale overthrusting of near surface layers previous quantitative analyses of mountain belts on venus have been restricted to thin skinned models6 8 but this style of deformation does not account for the pronounced topographic highs at the plateau edge we propose that the long wavelength topography of these venusian fold and thrust belts is more readily explained by horizontal shortening of a laterally heterogeneous lithosphere in this thick skinned model deformation within the mechanically strong outer layer of venus controls mountain building our results suggest that lateral variations in either the thermal or mechanical structure of the interior provide a mechanism for focusing deformation due to convergent global scale forces on venus

mapping the interactions between rivers and sand dunes implications for fluvial and aeolian geomorphology article i nfo the interaction between fluvial and aeolian processes can significantly change earth surface morphology when riversandsanddunes meet theinteractionofsedimenttransportbetweenthetwosystemscanleadtochangein eitherorboth systems however these two systemsare usuallystudied independently which leavesmanyques tions unresolved in terms of how they interact this paper carries out a global inventory using satellite imagery to identify 230 sites where there are significant fluvial aeolian interactions at each location key attributes such as wind river direction net sand transport direction fluvial aeolian meeting angle dune type and river channel pattern were identifiedand relationshipsbetween each factorwere analysed from thesedata six different types of interaction were classified that reflect a shift in dominance between the fluvial and aeolian systems results from this classification confirm that only certain types of interaction were significant the meeting angle and dunetype themeeting angle and interaction type and finally thechannel pattern andinteraction type however the findings also indicate the difficulties of classifying dynamic geomorphic systems from snapshot satellite images

grain shape variation in the st peter sandstone a record of eolian and fluvial sedimentation of an early paleozoic cratonic sheet sand abstract a combination of fourier grain shape analysis and scanning electron microscopy was employed in a sedimentological study of 317 samples of the st peter sandstone from 3 thick sections of the formation in southeastern minnesota the quartz grain shape analysis indicated that two grain roundness types are mixed in various proportions in all the samples studied the first type is a population of extremely smooth and well rounded grains whereas the second is a population of irregular angular grains scanning electron microscope examination of quartz surface textures revealed that the first grain roundness type was created during a stage of eolian transport and abrasion whereas the second grain roundness type is relatively unabraded and was probably transported in less abrasive fluvial nvironments with little or no residence time in subaerial environments as a result the latter grain roundness type still retains clearly defined relict overgrowths on their surfaces features inherited from the source of the st peter the relative proportions of these two grain roundness types vary in a nonrandom manner through the sections defining alternating units of abraded sand rich and unabraded sand rich beds this alternation found in beds deposited in a shallow marine environment probably reflects alternation in the immediate local source of the marine sand sheet the first source being coastal dunes and the second fluvial deltaic deposits upon consideration of the thickness of these beds and of the formation as a whole it is proposed that the bulk of the sediment in the st peter was transported from coastal eolian and fluvial deltaic sources into the deeper waters of the inner shelf by a combination of shoreface and river mouth bypassing mechanisms

dating of the upper pleistocene holocene volcanic activity of la palma using the unspiked k ar technique abstract combined unspiked k ar dating and detailed mapping have established the eruptive chronology of the cumbre vieja volcano in the canary islands the cv volcano is a large polygenetic volcano subaerial area 220 km2 subaerial volume 125 km3 elevation above sea level 2000 m which rests on the southern flank of the older taburiente cumbre nueva volcano we defined two stratigraphic units by their relationship to the development of coastal cliffs developed around the cumbre vieja volcano a cliff forming series and a scree forming series which is draped over the cliffs and forms the coastal platforms at their bases k ar ages were measured from 20 samples from the two series there are no discrepancies between k ar ages and the stratigraphy the k ar ages of one flow compare well with 14 c determinations 14 c 3 2 0 1 ka k ar 4 2 ka the ages of the scree forming series range from 3 2 ka to 21 2 ka the cliff forming series ages range from 20 2 ka to 123 3 ka but as the base of the sequence is not exposed the growth of the cumbre vieja volcano must have begun substantially earlier the boundary between the two coincides with the low sea level stand in the last glacial maximum which we infer to have been a period of intense coastal erosion dated samples from the upper part of the taburiente cumbre nueva volcano range in age from 853 10 ka to 566 8 ka the age of the youngest taburiente cumbre nueva volcano sample provides an upper bound on the age of the subsequent cumbre nueva giant lateral collapse

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

relative sea level changes and the development of a cambrian transgression the lower cambrian in northwest scotland is one example of a lower palaeozoic orthoquartzite carbonate succession deposited on a slowly subsiding peneplained precambrian basement during a period of relative sea level rise this particular setting led to the development of a very wide low gradient shelf which was extremely sensitive to minor sea level changes the basal quartz arenite section lower member pipe rock is a transgressive tide dominated systems tract but lacks a systematic parasequence architecture because of three factors a fluvial sediment flux was insufficient to induce shoreline progradation accommodation space was limited during sea level falls which are commonly expressed by widespread erosional surfaces and sediment yield to the shelf by transgressive reworking was a major contributor towards the preserved stratigraphy the storm dominated fucoid beds represent a condensed section and also show the effects of rapid and widespread facies belt oscillations because of the low shelf gradient an overlying highstand systems tract is also lacking partly due to the absence of a large fluvial sediment yield and also due to lowstand and transgressive reworking an erosively based tidal sandsheet at the top of the fucoid beds interpreted to be a lowstand systems tract therefore rests directly on the condensed section of the underlying sequence this was in turn reworked into linear tidal sandbanks salterella grit during slow sea level rise prior to the next major transgression the limited accommodation space therefore introduced a preservational bias towards deepening upward trends on a parasequence and sequence scale the oscillations in facies belts episodic subareal exposure and the potential to remove substantial portions of systems tracts suggests that lower palaeozoic orthoquartzite successions may exhibit regular and abrupt vertical shifts in depositional environment which given their subtle lithological character may require detailed analysis to identify such successions may also display incomplete development of several components of transgressive regressive sequence architecture

palaeosecular variation over the past 5 myr based on a new generalized database summary a generalized database of global palaeomagnetic data from 3719 lava flows and thin dykes of age 0 5 ma has been constructed for use with a relational database management system the database includes all data whose virtual geomagnetic poles vgp lie within 45 of the spin axis and can be used for studies of palaeosecular variation and for geomagnetic field modelling because many of these data were collected and processed more than 15 20 years ago each result has been characterized according to the demagnetization procedures carried out analysis of these data in terms of the latitude variation of the angular dispersion of vgps palaeosecular variation from lavas strongly suggests that careful data selection is required and that many of the older studies may need to be redone using more modern methods differences between the angular dispersions for separate normal and reverse polarity data sets confirm that many older studies have not been adequately cleaned magnetically therefore the use of the database for geomagnetic field modelling should be carried out with some caution using a vgp cut off angle that varies with latitude the best data set consists of 2636 results that show a smooth increase of vgp angular dispersion with latitude model g for palaeosecular variation which is based on modelling of the antisymmetric dipole and symmetric quadrupole dynamo families provides a good fit to these results

global geomagnetic field models for the past 3000 years transient or permanent flux lobes psvmod1 0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts lava flows and lake sediments at 24 sites evaluated at 100 year intervals from 1000 bc to ad 1800 we estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time intervals of overlap and use the 100 year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models global predictions of radial magnetic field at the coremantle boundary cmb as well as inclination and declination anomalies at the earth9s surface provide an unprecedented view of geomagnetic secular variations over the past 3000 years and demonstrate a consistent evolution of the field with time resolution of the models is poorest in the southern hemisphere where only six of the 24 sites are located several with incomplete temporal coverage low flux regions seen in the historical field near the north pole are poorly resolved but the northern hemisphere flux lobes are clearly visible in the models these lobes are not fixed in position and intensity but they only rarely venture into the pacific hemisphere the pacific region is seen to have experienced significant secular variation a strong negative inclination anomaly in the region like that seen in 0 5 ma models persists from 1000 bc until ad 1000 and then gradually evolves into the smaller positive anomaly seen today on average between 1000 bc and ad 1800 the non axial dipole contribution to the radial magnetic field at the core mantle boundary is largest in the north central pacific and beneath central asia with clear non zonal contributions at the earth9s surface average inclination anomalies are large and negative in the central pacific and most positive slightly to the east of central africa inclination anomalies decrease with increasing latitude average declinations are smallest in equatorial regions again with strong longitudinal variations largest negative departures are centred over australia and eastern asia secular variation at the earth9s surface is quantified by standard deviation of inclination and declination about their average values and at the cmb by standard deviation in radial magnetic field all three show significant geographical variations but appear incompatible with the idea that secular variation in the pacific hemisphere is permanently attenuated by greatly enhanced conductivity in d 00 beneath the region

remagnetization in lava flows recording pretransitional directions basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of la palma are characterized by overlapping components with very high unblocking temperatures a medium high temperature mht component with normal polarity is resistant up to 500 550 c while in most cases the high temperature ht primary reverse component cannot be isolated below 520 c the same situation with opposite polarities is observed also in a lava flow associated with the onset of the upper reunion reversal in the gamarri sequence in ethiopia the presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling the direction of mht is either close or similar to the direction of the ht component of the overlying flows which indicates that remagnetization occurred shortly after emplacement rock magnetic studies microscopic observations microprobe analyses and remagnetization experiments have been carried out to investigate the origin of mht the rock magnetic parameters do not exhibit specific anomalies the unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling the most plausible scenario is that baking by the overlying flows was accompanied by low temperature oxidation of titanomagnetite to cation deficient titanomagnetite this description also includes the presence of cracks fractures and other heterogeneities of the lava which induce variability between parallel profiles similarities with characteristics observed at steens mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

geomagnetic changes across the last reversal recorded in lava flows from la palma canary islands two adjacent sequences of 41 and 21 individual lava flows have been sampled in the island of la palma although no detailed correlation can be established between the two sections they both encompassed the matuyama brunhes m b reversal stepwise alternating field af and thermal demagnetization in air and in vacuum accompanied by investigations of the rock magnetic properties allowed us to identify several lava flows with the same signature and thus most likely associated with very short episodes of intense volcanic activity the intermediate virtual geomagnetic pole vgp positions do not cluster within any preferred geographic areas the polarity change is followed by a rebound or an excursion with a cluster of vgps over northeastern india whether or not this feature is truly associated with the transition or correlated with the delta event 0 69 ma will hopefully be resolved by further dating paleointensity experiments were performed on 60 samples using the modified thellier method coe 1967 corrections for magnetomineralogical changes induced by heating were applied using the technique of partial thermoremanent magnetization ptrm checks valet et al 1994b with a significant increase in the success rate from 38 to 58 twenty four independent estimates of the paleofield range between 3 9 and 46 0 t the field intensity was reduced to at least one tenth of the present day value during the transition low transitional field intensities are also observed during the rebound or excursion the lava flows preceding the polarity change show evidence for a significant drop in the dipole field intensity while the posttransitional flows exhibit high field intensities these features are compatible with the asymmetrical saw tooth pattern recently observed in sedimentary records meynadier et al 1994 the characteristics of the m b transition emerging from a compilation of the four available volcanic records are not inconsistent with the hypothesis of rapidly time varying nondipole transitional components favored by the decrease of the dipole and its subsequent recovery in the opposite polarity state

eolian deposits of the northern margin of the south china jianghan basin reconstruction of the late cretaceous east asian landscape in central china abstract the late cretaceous honghuatao formation includes a succession of thick clastic sedimentary sequences which are widely exposed in northwestern jianghan basin central china here detailed sedimentological and facies architectural analyses are performed allowing to re interpret and identify the controversy for sedimentary environment and to investigate the paleoclimatic condition the honghuatao stratigraphic sequence includes eolian dunes and dry interdunes deposits with fluvial channel deposits at the bottom and top the vertical change between fluvial and eolian systems indicates alternating wet and dry climate at the same time the provenance analysis combined with the paleocurrents from the w nw and ne indicates that huangling dome south qinling belt and dabie orogen supply the main detrital source for the honghuatao formation the late cretaceous first rifting subsidence in the jianghan basin provided accommodation space for the preservation of the aeolian accumulation moreover the existence of the mountain belts around the basin that is the orographic effect precluded the winds from transporting humidity to the interior of the jianghan basin generating the desert system this study shows that the formation of eolian deposits in the honghuatao formation and coeval strata in numerous adjacent basins was a sedimentary response to the prevailing desert climate in the mid to low latitude areas of the northern hemisphere during the late cretaceous which was predominately controlled by the subtropical high pressure system on a planetary scale

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

constructive and destructive episodes in the building of a young oceanic island la palma canary islands and genesis of the caldera de taburiente the results of new field observations 23 new k ar determinations and sixteen previously published determinations provide the basis for the reconstruction of the subaerial volcanic history of the island of la palma after the seamount activity represented by the materials of the basal complex an eruptive phase between 2 0 and 1 3 ma formed a large shield a period of volcanic quiescence followed until around 1 ma during which a large lateral collapse partly destroyed the former edifice between 1 05 and 0 7 ma activity was renewed in the shield and a n s ridge was built in the southern part of the island around 0 7 ma two new large lateral collapses affected the western part of both edifices and they were followed by eruptions between 0 71 and 0 65 ma which built a new edifice that partly filled the depressions thus created the caldera de taburiente constitutes the eroded remnants of the depression formed in the northern shield from 0 65 ma to present activity has been restricted to the n s ridge which has continued to grow southwards there was a general n s migration of volcanic activity with time but in the shield the trend was northwest to southeast eruptive rates seem to have been fairly constant during the different eruptive phases considered between 0 15 and 0 37 km3 ka a very similar succession of constructive and destructive episodes has been obtained for the neighboring island of hierro but in this case the activity started around 0 8 ma and eruptive rates were about 0 5 km3 ka

later stages of volcanic evolution of la palma canary islands rift evolution giant landslides and the genesis of the caldera de taburiente the islands of la palma and el hierro form the western end of the hotspot induced canary islands chain both islands are at present in the earliest and fastest shield building stage of growth and show many similarities with the hawaiian islands la palma shows two very distinct phases of volcanic construction 1 a pliocene submarine volcanic and intrusive series interpreted as an uplifted seamount at least 1500 m above present sea level and 2 a subaerial volcanic series erupted in the past 2 m y taburiente volcano initially formed the northern part of the island and then extended to the south forming a ridge cumbre nueva ridge that was partially destroyed about 560 ka by a giant lateral collapse the cumbre nueva collapse possibly involving 180 200 km 3 of subaerial volcanic material the northwest boundary of the cumbre nueva collapse formed a vertical scarp that enlarged by headward erosion formed the spectacular depression of caldera de taburiente cumbre vieja volcano a north south elongated rift forms the southern half of la palma and constitutes its last stage of growth including all eruptive activity in the past 125 k y detailed field observations mapping and high precision radiometric dating have allowed reconstruction of the growth and structural changes of the cumbre vieja volcano interactions between eruption rates coastal erosion and glacio eustatic sea level change allow stratigraphic subdivision of the edifice the structure of the volcano has been controlled for most of its history by a rift system with a clearly dominant north south and less distinct north east and north west volcanic rift zones the rift reorganization of the volcano to a single north south rift since 7 ka and the opening of faults during the 1949 eruption probably reflect increasing instability of the west flank of the volcano

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

sedimentology and diagenesis of the st peter sandstone central michigan basin united states the middle ordovician st peter sandstone occurs at depths between 1600 and 3600 m in the central michigan basin integration of conventional core and wireline log studies indicates that the formation consists of from base to top 1 up to 250 m of sandstone deposited in intertidal and supratidal sand flats with associated dolomitic lagoonal deposits and shallow subtidal shoreface environments 2 sandstones deposited in subtidal shoreface to upper offshore environments and 3 dolomitic and argillaceous sands deposited in a storm dominated epeiric sea sandstone composition is closely related to depositional environment quartz arenites occur in higher energy littoral facies whereas feldspathic and carbonate rich sands occur in predominantly lower energy shelfal f cies important modification of primary mineralogy and porosity occurred during diagenesis a generalized basinwide model for paragenesis includes 1 early calcite marine cement 2 syndepositional dolomitization 3 widespread precipitation of quartz and k feldspar overgrowths 4 pervasive replacement of early carbonate by burial dolomite 5 local dissolution of unstable framework grains e g k feldspar and carbonate cement 6 growth of authigenic clay and 7 pressure solution and additional precipitation of quartz overgrowths authigenic clays apparently formed along with or subsequent to economically significant secondary porosity the different pathways of sandstone diagenesis observed in the st peter sandstone are largely dependent on primary textures and composition inherited from the environment of deposition rather than other factors such as depth of burial or position in the basin the relationships among primary textures and composition diagenesis and reservoir sandstone properties is useful for prediction of reservoir quality in the st peter sandstone in the michigan subsurface weakly cemented sandstone reservoirs characteristic of high energy shoreface and shallow offshore depositional environments have the best reservoir quality in the basin clay cemented sandstone reservoirs deposited on offshore shelves have the poorest reservoir quality incompletely quartz cemented sandstone reservoirs comm n in intertidal supratidal facies have intermediate reservoir quality

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

parent material influence on soil distribution and genesis in a paleudult and kandiudult complex southeastern usa parent materials greatly influence soil development and the distribution of soils on the southeastern us coastal plain we examined the physical chemical and mineralogical properties of 11 pedons in a 1 ha plot on the upper coastal plain of georgia usa uniformity of parent materials was assessed by sand grain size characteristics the soils have sandy epipedons of variable thickness and argillic horizons of variable texture six of the pedons also have kandic horizons they are classified us soil taxonomy in psammentic grossarenic arenic and typic subgroups of paleudults and kandiudults loamy pedons possess argillic horizons with two distinct increases in clay and greater differences between eluvial and illuvial horizons than sandy pedons the upper boundary of the argillic horizon is approximately parallel to the present geomorphic surface suggesting that it is associated with the contemporary surface discontinuities identified by changes in sand grain size ratios and plots of the third skewness and fourth kurtosis moments of sand grain distribution roughly correspond to the bottom of the solum our data suggest that there are both eolian and fluvial components in the solum whereas subjacent horizons are completely derived from fluvial deposits sandier pedons have greater gibbsite kaolinite ratios possibly because greater permeability has enhanced leaching and si loss our data suggest parent materials largely control soil distribution over this plot

persistently anomalous pacific geomagnetic fields a new average geomagnetic field model for the past 3kyr als3k helps bridge a large temporal sampling gap between historical models and more traditional paleo magnetic studies spanning the last 5 myr a quasi static feature seen historically in the central pacific has the oppo site sign in als3k its structure is similar to but of larger amplitude than that in the time averaged geomagnetic field for the last 5 myr anomalous geomagnetic fields exist be neath the pacific over timescales rangingfrom 10 2 10 6 years it is unlikely that bias over such long time scales arises from electromagnetic screening but conceivable that the lorentz force is influenced by long wavelength thermal variations and or localized regions of increased electrical conductiv ity associated with compositional anomalies and possibly partial melt this is consistent with recent seismic obser vations of the lower mantle

k ar ages and magnetic stratigraphy of a hotspot induced fast grown oceanic island el hierro canary islands abstract the combined use of accurate radiometric dating and magnetic stratigraphy can be applied to define the main stages of the building of oceanic volcanic islands this method has been successfully applied on the island of el hierro the westernmost island of the canaries archipelago for the emerged part of this island built in the last 1 2 ma magnetic stratigraphy and mapping show the presence of four consecutive subchrons k ar dating of eighteen samples in stratigraphic sequences using an unspiked technique produced ages consistent with the general volcanic stratigraphy the ages show that the magnetic polarity zones correlate to the upper part of the matuyama and to the brunhes chrons the jaramillo normal polarity subchron of the upper part of the matuyama reverse polarity chron is recorded in one of the lava sequences the lower limit of this event is constrained by two ages of 1 04 0 02 ma and one of 1 05 0 02 ma close to the value deduced from the astronomically tuned magnetic polarity time scale apts the volcanic history can be summarised by two consecutive main basaltic volcanic edifices the el tinor active during the upper part of the matuyama chron and the el golfo edifice constructed during the brunhes chron

persistent patterns in the geomagnetic field over the past 2 5 myr historical geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations lobes at the surface of the liquid core and regions of rapid change extending from the atlantic to the indian ocean1 palaeomagnetic data define a time average over several thousand years2 which might reflect the stationary parts of the present field but unfortunately the historical record is too short to provide a satisfactory average3 here we model palaeomagnetic directions from the past 2 5 myr using the same methods as for modern data4 and find the two northern lobes in the same position as today over arctic canada and siberia in southern regions by contrast the field appears to have been smoothed out as might be expected from the current rapid secular variation1 we propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years as would occur if the solid mantle controls flow at the top of the core

anomalously shallow palaeomagnetic inclinations and the question of the age of the canarian archipelago summary extensive geological geophysical and geochronological data available from the canary islands establish conclusively that formation of the oldest subaerial volcanic structure of the islands began during the miocene a mid cretaceous age for these volcanic formations has been postulated in previous works on the basis of palaeomagnetic determinations the results obtained in the present palaeomagnetic study of lanzarote include analysis of the record of variations of magnetic inclination in boreholes that penetrate the whole of the oldest volcanic series they show that the excessive age previously assigned to these formations is due to the utilization of volcanic units with abnormally low 15 magnetic inclinations lgis in lanzarote lavas exhibiting lgis appear interbedded in a volcanic series that overall shows a typical miocene inclination 45 the units stratigraphically beneath and above the lgi horizons give in fact directions consistent with the middle upper miocene field direction d 359 i 45 with k 29 a95 6 7 and a palaeopole of 87 n 178 e thereby confirming the miocene age of the oldest subaerial volcanics of lanzarote short excursions of the geomagnetic field seem likely to be the explanation for these lgis since other factors such as tectonic tilting post eruptive modification of the primary remanence or errors in sample orientation can be disregarded the detection in the canary archipelago of volcanic units with abnormally low magnetic inclinations seems to be related to the relatively continuous record of the geomagnetic field in rapidly growing volcanic edifices as seems to be indicated by the presence of a few usually one or two short polarity events in volcanic suites of several hundred metres thickness

a composite tectonic eustatic origin for shelf sandstones at the cambrian ordovician boundary in north greenland shelf sandstones deposited at the cambrian ordovician boundary in central and western north greenland have a sheet like large scale geometry and extend for over 300 km along strike within a shallow marine carbonate dominated platform sequence conodont faunas from units above and below the most extensive sand sheet the kap coppinger member of the cass fjord formation and the permin land formation indicate that deposition of the sands spanned the proavus intermedius and lindstromi conodont biozones sand was derived by reworking of older quartzite dominated formations following the uplift and unroofing of a platform bulge to the east during the cambrian this sediment was transported northwards from the uplifted area across an unstable platform into a deep water trough however during sea level lowstands within the cambrian ordovician boundary interval this clastic material was deflected westwards and transported across the stable platform by progradation of tide and storm dominated shorelines facies sequences in the thickest part of the permin land formation suggest at least three such regressive cycles combined to form a composite sand body each regressive cycle comprises a shallowing upward transition from tide and storm deposited marine sediments to clean sandstones of marine or terrestrial origin the cycles are separated by thin but laterally extensive dolomites interpreted to represent drowning events thinner sequences further to the east are attributed to reduced accommodation space as a consequence of lower subsidence rates close to the uplifted source area whilst thin sequences in the west are ascribed to the effect of increasing distance from the sediment source so that only the youngest of the three regressive cycles is present

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

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

paleomagnetism and 40ar 39ar ages from la palma in the canary islands 1 the structure of the time averaged geomagnetic field has been known forcenturies to be approximately dipolar significant departures of the time averagedfield from that of an axial geocentric dipole however have been reportedfor decades the data on which time averaged field models are based must beof the highest quality in order to document subtle long term features wepresent here new paleomagnetic data and 40ar 39ar ages for the island of la palma the canary islands paleomagnetic samples were obtained from 28 lava flows of these 21 met ourminimum acceptance criteria for use in time averaged field models the 40ar 39ar age determinationswere successfully carried out on samples from eight of the flows our isotopicages and paleomagnetic polarities are consistent with the currently acceptedgeomagnetic reversal timescales the reversed data in the updated databaseare antipodal to the normal data within the uncertainties and the time averageddirection is indistinguishable from that expected from a geocentric axialdipole

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity