eustatic sea level record for the cenomanian late cretaceous extension to the western interior basin usa a combination of biostratigraphic markers ammonites inoceramid bivalves and carbon isotope excursions is employed to establish a high resolution correlation between the middle to late cenomanian successions of the western interior basin usa and the anglo paris basin southern uk sequences identified from sedimentologic criteria in the pueblo succession and elsewhere in the western interior basin are shown to coincide precisely with globally recognized sea level events and were therefore under eustatic control this evidence refutes arguments that cenomanian sequences in the western interior basin were formed by local tectonic events the interaction of longer term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries a crossplot of radiometric ages derived from north american bentonites against an orbitally tuned time scale developed in the anglo paris basin provides support for the argument that the sequences were controlled by the 405 k y long eccentricity cycle

magnetism and transmission electron microscopy of fe ti oxides and pyroxenes in a granulite from lofoten norway granulites from southwest lofoten norway often carry intense and stable components of natural remanent magnetization nrm coercivities in excess of 100 mt and blocking temperatures up to 580 645 c are common several phases in these rocks can carry stable components of nrm clinopyroxenes augite with exsolved pigeonite and orthopyroxene usually contain abundant fe ti oxides the majority of which are too small to be studied conclusively with visible light additionally these rocks typically contain coarse grained 100 1000 m exsolution intergrowths of rhombohedral fe ti oxides and coarse grained magnetite we have studied the magnetism and mineralogy of the titanohematite and the inclusion bearing augites in one granulite sample using transmission electron microscopy tem and magnetic experiments to understand better the remanence properties of naturally occurring fe ti oxides and their relationships to silicate minerals the coarse grained titanohematite has a coercivity of 350 mt and a laboratory saturation remanence that unblocks at 570 600 c the hysteresis characteristics of this titanohematite are typical of much smaller 10 m sized single domains which apparently reflects the presence of exsolved ilmenite lamellae as small as 10 nm in thickness these lamellae presumably inhibit 1 domain wall nucleation and 2 movement of walls that manage to nucleate the inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560 c magnetite hematite and ilmenite are present in the augite and intergrowth of the rhombohedral phases revealed by tem persists down to the 20 nm scale the distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure while that of the rhombohedral fe ti oxides is strongly controlled by silicate microstructure with intergrowths of these oxides commonly decorating lamellae of pigeonite orthopyroxene and clinoamphibole within the augite the nrm of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m y ago paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event in addition to the included oxides the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites pigeonite lamellae are not parallel to the b crystallographic axis pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur rare earth elements are concentrated within inclusions that reside in the augite

interpreting avulsion process from ancient alluvial sequences guadalope matarranya system northern spain and wasatch formation western colorado alluvial deposits of the guadalope matarranya system oligocene ebro basin spain and the wasatch formation eocene western colorado provide time integrated records of the process of river channel avulsion these sequences consist of isolated channel belt sandstones incised into and abruptly overlain by flood plain siltstones indicating deposition by avulsive river systems the geometry and distribution of channel incisions suggest that avulsion was not controlled by tectonics climate or base level changes but formed by autocyclic processes measurements from 221 channel fills in the guadalope matarranya system and 38 from the wasatch formation allow us to statistically characterize channel geometries we infer to be associated with establishment and abandonment of individual river avulsions paleoflow depths in both systems average 1 4 to 1 6 m aggradation height superelevation of channel margin levees are on average 0 6 and 1 1 times paleoflow depth in the guadalope matarranya and wasatch systems respectively these results are consistent with values from recently avulsed modern rivers and suggest that 1 flow depth is the appropriate parameter against which to scale the critical superelevation necessary for channel avulsion and 2 the increase in potential energy due to channel perching drives the lateral instability that is needed for avulsion to be successful numerous stacked channel fills indicate repeated reoccupation of the same site by avulsing channels these reoccupation channels indicate that inherited flood plain topography here abandoned channel forms was an important control on the arrival site of newly avulsed channels comparison of our results to others suggests two end member types of avulsion can take place incisional avulsion seen here is characterized by an early incision phase followed by infilling by migrating bar forms aggradational avulsion begins with aggradation followed in time by stream integration into a single downcutting channel we suggest that the type of avulsion is strongly influenced by whether or not the adjacent flood plain is well or poorly drained in both cases subsequent aggradation and channel perching increase the chances that some triggering event will lead to avulsion

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

thermoremanent magnetization carried by synthetic analogues of the altered olivines of igneous rocks abstract the deuteric alteration of olivines in nature has been simulated in the laboratory by heating members of the forsterite fayalite solid solution at temperatures between 850 c and 1280 c and under oxygen fugacities between 10 13 1 and 10 5 7 atmospheres magnetite was produced analogous to the dendritic and symplectite intergrowths found in nature the thermoremanent magnetization trm acquired by the magnetite was intense and stable and was found to correlate with the temperature tox at which the deuteric alteration had been carried out in the lowertox range 850 950 c trm intensity increased with increasingtox and this may be a rare example of trm intensity increasing with increasing grain size as expected from the monodomain model the highest intensities are higher than that of any magnetite trm recorded in the literature applying the monodomain model to the highertox range 1000 1200 c yields implausible values for particle volume whereas the application of multidomain models yields plausible demagnetizing factors discrepancies between the trm acquisition curves and the temperature dependence of coercive force point to the limitations of the simple multidomain model

analysis and modelling of the aeromagnetic anomalies of gran canaria canary islands abstract in this paper we present a crustal magnetic model of the volcanic island of gran canaria based on aeromagnetic data this magnetic study has made it possible to constrain the location and geometry of 1 the mafic core of gran canaria built as the result of the crystallization of magmas which rose from the mantle during the submarine and shield basaltic stages of its evolution the most intense magnetic high displayed over the nw part of the island likely shows the main feeding system of the shield volcano 2 the residual syenitic shallow magma chamber which fed the salic volcanic activity in the center of the island linked to the formation of the tejeda caldera and to the intrusion of felsic rocks which made up a cone sheet 3 a reversely magnetized linear intrusion located in the marine area to the nw of the island which could be related to the magmatic source of the submarine volcanism between gran canaria and tenerife most of these magnetic sources show a linear pattern with trends that are in close agreement with the orientations of previously identified fractures the magmatic intrusion to the nw of the island could be related with a wsw ene active fault between gran canaria and tenerife while the main trend and location of the mafic core agree with a nw se fault suggested by geological studies this means that these magnetic sources are the result of the ascent and intrusion of magma along regional fractures therefore this study provides fresh data which demonstrate the influence of regional tectonics on the growth of gran canaria during its entire evolution finally the spectral analysis of the magnetic anomaly map suggests that it is possible that rocks located at mantle like depths from the moho to about 23 km behave as magnetic sources a fact that can be related with the magmatic underplating detected by other geophysical techniques

source of stable remanence in chromite ores rock magnetic investigations of both the natural chromite ores and their purified chromite concentrate mg 6fe 4cr1 4al 6o4 were carried out these include measurement of natural and saturation remanent magnetism and their stability with respect to alternating field and thermal demagnetisation irm acquisition studies saturation magnetisation versus temperature studies and high field 1000 mt hysteresis characteristics from these studies it was observed that the medium high coercivity component of remanence is associated with the chromite minerals and is carried largely or entirely by them therefore it is concluded that chromites carry permanent magnetisation of palaeomagnetic significance however this result is in sharp contrast to the assumption in the literature that the spinels of the above composition are non magnetic the study has also demonstrated that the magnetic properties of artificial fe cr spinels do not conform to their natural analogues reasons for the discrepancy have been postulated

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

domain size closure domains and the importance of magnetostriction in magnetite abstract previous micromagnetic calculations invoking magnetostatic exchange and crystalline anisotropy energies predict about an order of magnitude more domains in magnetite than have experimentally been observed domains observed on magnetite by the magneto optic kerr effect are of similar size as previously observed ones by the bitter technique simple but quite accurate domain calculations show that for rectangular parallelepipeds of magnetite it is the closure four domain configuration that should be of lowest energy for all multidomain grain sizes unless magnetostrictive energies are considered for the observed domain pattern it is shown that the domains themselves must carry significant magnetoelastic energies

oriented inclusions of magnetite in clinopyroxene source of stable remanent magnetization in gabbros of the messum complex namibia 1 crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the early cretaceous messum complex namibia rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single domain magnetite consistent with the observed sizes and shape anisotropy of the magnetite inclusions as in previous studies of similar inclusions these are inferred to have formed by exsolution two arrays of inclusions are regularly present in the messum clinopyroxenes inclined at about 74 consistent with formation at about 800 c deduced from optimization of phase boundary orientations virtual geomagnetic poles from these rocks are consistent with reference data confirming that the magnetization is of thermoremanent origin bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

magnetic mineralogy of sandstones from the lunde formation late triassic northern north sea uk origin of the palaeomagnetic signal the minerals responsible for the magnetic properties of the late triassic lunde formation have been examined using a combination of magnetic mineral extraction microscopy and rock magnetic measurements the magnetic mineralogy is controlled by the depositional environment and the effects of diagenesis by a process of differential preservation and modification of the original detrital minerals the magnetic susceptibility and remanence properties of the sediments are strongly influenced by the proportion of silt and clay sized clastic material which contains most of the magnetic minerals paramagnetic minerals both of depositional origin and produced during diagenesis are largely responsible for the magnetic susceptibility a suite of residual ferrimagnetic oxides chromite and mn substituted magnetite is responsible for the remanence properties in grey sandstones because most of the originally deposited magnetic fe ti oxides have been removed by reducing conditions during diagenesis however a small number of reddened horizons preserve a higher proportion of original detrital fe ti oxides ferrimagnetic ilmenite ferrimagnetic chromite and magnetite although authigenic hematite has replaced most of the original mineralogy in the reddened horizons magnetic oxide inclusions in quartz and feldspars are important contributors to the remanence signal particularly in horizons which have a low content of discrete detrital ferrimagnetic grains either through diagenetic dissolution or paucity of the silt and clay fraction within the grey sandstones the triassic palaeomagnetic signal is carried by magnetite as inclusions in detrital silicates with a contribution from detrital ferrimagnetic chromite a model of the effects of diagenesis on the magnetic mineralogy in deeply buried sequences is proposed

40ar 39ar incremental heating studies on the tudor gabbro grenville province ontario its bearing on the north american apparent polar wander path in late proterozoic times summary 40 ar 39 ar incremental heating studies have been carried out on samples taken from the tudor gabbro grenville province ontario in an earlier k ar study these rocks have yielded an isochron age of 700ma together with very high initial argon ratios age spectrum plots on whole rock samples in general display a saddle shaped character with two of them exhibiting minima close to 700 ma no clear plateaus are observed for these rocks a hornblende separate records the time 1110 ma at which the stock finally cooled through the 590 c isotherm the tudor gabbro was prob ably intruded into an area undergoing middle amphibolite facies meta morphism about 1180 ma ago the age spectra of two whole rock samples together with that of their plagioclase separates suggest that the stock cooled to 200 250 c at about 720 ma slow cooling averaging about 1 o c ma 1 is indicated for this section of the grenville province for the period 1100 700 ma if the age of the tudor gabbro s palaeomagnetic pole position is taken to be 720 ma the hadrynian track hypothesis leads to very high polar wander rates of 20 em a 1 for the period 820 720 ma if this hypo thesis is rejected the average drift rate for this period would be 4 em a 1 in much better agreement with published values of 5 em a i for the period 1400 820ma

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

magnitude and timing of peat to coal compaction the peat to coal transition is commonly assumed to be accompanied by compaction that decreases the thickness of the organic deposit to values of 10 or less of the original peat thickness decompaction modeling using such values for coal seams in contact with penecontemporaneous channel sandstones leads to impossible depositional geometries for the sandstones which have a final thickness that is 90 of their original thickness similarly decompaction modeling of the fragments of organic material within channel lags using the assumed large peat compaction value results in the destruction of the associated sedimentary structures such as trough cross beds dinosaur tracks in the roofs of coal mines show a shallow depth of penetration and a preservation of foot morphology that is not possible unless the peat the animals walked upon was very firm these data confirm the macroscopic observations from coal balls that the compaction associated with the peat to coal transformation is minimal equally important the data also show that most of the compaction occurs essentially at the surface and not at the depths where coalification takes place

complexity in a cellular model of river avulsion abstract we propose a new model of river avulsion that emphasizes simplicity self organization and unprogrammed behavior rather than detailed simulation the model runs on a fixed cellular grid and tracks two elevations in each cell a high elevation representing the channel levee top and a low one representing the channel bottom the channel aggrades in place until a superelevation threshold for avulsion is met after an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation flow path depends sensitively on floodplain topography particularly the presence of former abandoned channels several behavioral characteristics emerge consistently from this simple model 1 a tendency of the active flow to switch among a small number of channel paths which we term the active channel set over extended periods leading to clustering and formation of multistory sand bodies in the resulting deposits 2 a tendency for avulsed channels to return to their previous paths so that new channel length tends to be generated in relatively short segments and 3 avulsion related sediment storage and release leading to pulsed sediment output even for constant input each of these behaviors is consistent with observations from depositional river systems a single valued threshold produces a wide variety of avulsion sizes and styles larger nodal avulsions are rarer because pre existing floodplain topography acts to steer flow back to the active channel channel stacking pattern is very sensitive to floodplain deposition this work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution

analysis of the aeromagnetic anomalies of the auca mahuida volcano patagonia argentina we present the analysis of the subsurface structure of the auca mahuida volcano based on high resolution aeromagnetic data integrated with the available geological information most of the detected magnetic anomalies have a dipolar structure opposite to that of the present geomagnetic field according to the available geochronological data and paleomagnetic measurements the source bodies of auca mahuida mainly emplaced in the matuyama reverse polarity chron the reduction to the pole map confirms that the magnetization direction is mainly reverse with only few anomalies normally magnetized two opposite coexisting polarities do not allow to fully remove the dipolar character of the field in the reduction to the pole transformation therefore we model the measured anomaly field by applying analytical techniques that are independent of the magnetization direction the obtained anomaly strikes and source geometries indicate an emplacement of intrusive bodies controlled by the regional faults affecting the auca mahuida basement and the sedimentary successions of the neuquen basin magma upraised along these faults and fractures feeding the volcanic activity and subsequently crystallized the averaged power spectrum and euler deconvolution indicate source depths consistent with those of the intrusions recognized in wells borehole data highlight the widespread presence of intrusive bodies below the auca mahuida central crater and the peripheral sectors at depth of 2 km below sea level these bodies have played a major role in the thermal maturation of hydrocarbons and in the subsequent accumulation of oil below the volcano the obtained results shed light on the auca mahuida feeding system and on the intrusions geometry also pointing out the effectiveness of the magnetic prospecting in the oil industry even in presence of strong remanent magnetization

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

electron microscopy of iron oxides and implications for the origin of magnetizations and rock magnetic properties of banded series rocks of the stillwater complex montana the origins of multiple magnetizations of the archean stillwater complex have been investigated through scanning electron microscopy and scanning transmission electron microscopy observations of mineralogical relations using representative samples from nine sites in mafic banded series rocks on the basis of directional grouping and demagnetization behavior three magnetizations here labeled a b and c have been recognized the natural remanent magnetizaton nrm is lypically dominated by only one of these magnetizations and multicomponent behavior in individual specimens is rare the a remanence resides in magnetic grains of high median destructive fields and high discrete laboratory unblocking temperatures and is inferred to be a primary thermoremanent magnetization of circa 2 71 ga age the b and c magnetizations of lower median destructive fields and more distributed unblocking temperatures are inferred to be secondary and related to alteration including serpentinization involving limited moderate to low temperature fluid interaction perhaps in response to thermotectonic events e g mafic dike emplacement samples with nrm dominated by the a magnetization contain titanium free magnetite needles width 1 m and maximum dimension between about 2 and 50 m with preferred orientations in cumulus plagioclase crystals these oxides are interpreted to have formed during initial crystallization of cumulus phases in samples containing only b or c magnetizations such needles are absent in cumulus plagioclase whereas secondary fe bearing silicates particularly zoisite and chlorite are conspicuous the samples dominated by the b or c magnetizations contain anhedral low chromian magnetite fecr2 xfexo4 1 5 x 2 0 along fractures in both cumulus plagioclase and pyroxene and in zones of more pervasive alteration of pyroxenes the paragenesis of these oxides is consistent with an origin related to hydrothermal alteration hysteresis data show that plagioclase crystals containing magnetite needles have higher remanent coercivities hcr 60 to 100 mt whereas plagioclase crystals with anhedral chromian magnetite have remanent coercivities less than 60 mt consistent with their alternating field response because oxygen isotope data show no indication of appreciable alteration of host plagioclase we infer that the absence of magnetite needles and their associated a magnetization are caused by selective alteration of the magnetite needles to nonmagnetic phases in the remagnetized gabbros in this case rock magnetic and paleomagnetic data appear to be higher resolution indicators of fluid rock interaction than the stable isotope data anhedral chromian magnetite precipitated during hydrothermal alteration and therefore carries secondary magnetizations

significance of channel belt clustering in alluvial basins the distribution of channel deposits in alluvial basins is commonly used to interpret past changes in climate tectonics and sea level here we present preliminary evidence that longtime scale 10 3 10 5 yr self organization in fl uvial systems may generate structured stratigraphic patterns spontaneously in the absence of or independent from changing basin boundary conditions a physical experiment and an ancient alluvial succession ferris formation latest cretaceous paleogene south central wyoming both show stratigraphy where clusters of many closely spaced channel deposits are separated from each other by extensive intervals of overbank mudstones analysis using spatial point process methods shows that channel deposits in both basins are statistically clustered over intermediate basin length scales in the experiment external controls base level subsidence rate and sediment water supplies were not varied and therefore not factors in cluster formation likewise the ancient system lacks stratigraphic and sedimentologic evidence of external controls on channel clustering we propose that channel clusters as seen in this study refl ect a scale of flself organization that is not usually recognized in ancient deposits this type of internally generated stratigraphy should be considered when reconstructing tectonic climate and sea level changes from

anomalous clastic wedge development during the sevier laramide transition north american cordilleran foreland basin usa new high resolution regional correlation and isopach maps provide evidence that 1 laramide style deformation began as early as ca 77 ma in central utah and 2 rapid 208 km m y 1 and extensive 400 km progradation of a clastic wedge was facilitated by reduced subsidence during the transition from sevier to laramide style deformation this study defi nes three campanian alluvialto marine clastic wedges that traversed 200 400 km eastward across the utah colorado segment of the cordilleran foreland basin wedges a and c are thicker successions with rising trajectory shoreline stacking patterns blackhawk formation and lower castlegate sandstone bluecastle tongue and rollins sandstone that refl ect relatively slow overall progradation 50 81 km m y 1 of narrow 10 20 km wide wave dominated shorelines in contrast wedge b consists of lower volume successions with a fl at to falling shoreline stacking pattern middle castlegate sandstone sego sandstone neslen formation corcoran and cozzette members of the iles formation that suggests rapid progradation 208 km m y 1 of embayed 60 80 km wide mixed energy wave and tide infl uenced shore lines wedges a and c prograded 200 250 km in more than 3 m y whereas wedge b prograded 340 400 km in 2 m y 170 km m y 1 the anomalously extensive wedge b is unique in the utah colorado segment because of its long extent rapid progradation rate dominance of tidally infl uenced facies long transit transgressions regressions and low accommodation nested sequence architecture stratigraphic relationships indicate development of wedge b coeval with both sevier and laramide style deformation in utah assuming a constant sediment supply the extensive 300 400 km sheet like amalgamated wedge wedge b may have been caused by 1 reduced subsidence driven by a fl exural interference pattern whereby two short wavelength 200 km fl exural profi les are superimposed during the uplift of both basement cored and thin skinned thrust belt 2 an increase in or eastward migration of dynamic subsidence during tectonic transitions associated with slab fl attening or rollback i e long wavelength fl exure or 3 reduced subsidence due to short wavelength fl exural interference augmented by the reduction or migration of long wavelength fl

magnetism of biotite crystals abstract we report mossbauer spectra magnetic hysteresis acquisition and demagnetization of isothermal remanent magnetization irm and low temperature irm warming curves of biotite crystals as well as a summary of domain observations on magnetic extracts the biotites are from 1 0 to 1 2 ga gneisses and 1 85 ga plutons of the grenville churchill and bear provinces canada most have paramagnetic susceptibilities in the range 0 45 1 2 10 3 si similar to reported values for iron rich biotites magnetite identified by its 120 k verwey transition is an inclusion in all the biotites sizes and domain states of inclusions correlate with magnetite content as measured by saturation magnetization ms the higher ms is the lower are the domain state indicators mrs ms mrs is saturation irm and coercive force hc added to a base population 0 003 0 1 of pseudo single domain psd grains with mrs ms from 0 1 to 0 3 and hc from 7 to 20 mt is 1 in two exceptional cases 7 and 14 of coarse grained multidomain md magnetite with much softer hysteresis properties the underlying psd magnetite fraction in biotites potentially retains useful paleomagnetic remanence in the more magnetic micas this stable fraction may be swamped by md magnetite with less stable behavior

palaeomagnetic study of the nagssugtoqidian mobile belt in central west greenland abstract this paper reports a palaeomagnetic study of amphibolite and granulite facies basement rocks across a 60 km wide north south traverse through the nagssugtoqidian mobile belt in west greenland which incorporates the c 2500 ma itivdleq shear belt the c 1800 ma ikertoq shear belt and adjoining terrains it also integrates the results with earlier investigations of the belt to produce a composite record of the magnetic field during uplift and cooling of the terrain through c 600 500 c at c 1750 1600 ma progressive a f and thermal cleaning of samples from 81 sites identifies a range of magnetic components lowest blocking temperature remanences in nagssugtoqidian c 1700 ma amphibolite facies rocks yield steep w to n positive components residing in magnetite granulite facies rocks formed at c 1800 ma generally have a dual record incorporating steep n to nw positive components unblocked by 580 c and shallow e w directed components of dual polarity only unblocked close to the curie point of haematite a common feature of relict c 2500 ma amphibolite and granulite facies terrains is a progressive movement of sample directions back along a palaeofield migration path with treatment and is interpreted to represent the recovery of a discrete spectrum acquired during a p w areas with a distributed spectra through the critical range of blocking temperatures altitude controlled sections and blocking temperature relationships all record consistent a p w during the interval of uplift related cooling the collective data define a clockwise palaeofield migration path between d 174 i 32 and d 348 i 66 referred to as the a magnetisation the equivalent palaeopoles define one limb of an a p w loop conforming to other laurentian poles assigned to the interval 1750 1650 ma and widely represented as a post hudsonian overprinting or cooling remanence shallow e w directions designated b predominate in late tectonic diorites and are therefore younger than 1700 ma they appear to represent a localised reheating event near the northern margin of the ikertoq shear belt possibly related to this magmatism which produced the hemoilmenite phases the equivalent palaeopoles are provisionally interpreted to represent part of the return path of the apw loop because they are coincident with other poles dated c 1630 ma collectively the palaeofield migration data from all studies define a thermal dome broadly coincident with the ikertoq shear belt but it is not possible to isolate the contribution of components of vertical movement along faults running through the belt to this regional effect

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

facies and architectural analysis to interpret avulsion style and variability upper cretaceous blackhawk formation wasatch plateau central utah u s a abstract sequence stratigraphic models emphasize the role of external allogenic controls such as relative sea level tectonic subsidence and climate on alluvial to coastal plain stratigraphic architecture at the expense of internal autogenic behaviors such as avulsion these models are tested via sedimentologic analysis of fine grained floodplain deposits and their relationship to channelized fluvial sandbodies in a well exposed alluvial to coastal plain succession developed under progressively increasing distance from the coeval shoreline and an accompanying decrease in the rate of accommodation creation late cretaceous blackhawk formation wasatch plateau central utah u s a the studied alluvial to coastal plain succession contains fourteen lithofacies which are grouped into four associations and eight architectural elements facies associations consist of 1 channelized fluvial sandstone bodies 2 nonchannelized fluvial sandstone and siltstone bodies 3 fine grained floodplain deposits and 4 brackish floodplain deposits paleosols are immature and are classified as entisols inceptisols and histosols integrated analysis of paleosol character facies analysis and architectural element analysis indicates deposition on a rapidly aggrading floodplain that was subject to frequent crevassing and channel belt switching the character relative abundance and vertical stacking of the paleosol types architectural elements and the first three facies associations exhibit little variation stratigraphically from base to top of the blackhawk formation c 250 m or paleogeographically between the northern and southern limits of the outcrop belt c 100 km local stratigraphic architectures record three styles of avulsion representing avulsion by reoccupation avulsion by progradation and avulsion by incision avulsion style displays no consistent pattern with stratigraphic position or paleogeographic location in combination these results suggest that floodplain sedimentation and local stratigraphic architecture are dominated by autogenic avulsion the three observed avulsion mechanisms operated in apparent uniformity over a range of tectonic subsidence rates c 80 700 m ma and distances from the coeval shoreline c 0 100 km

evaluation of sediment provenance using magnetic mineral inclusions in clastic silicates comparison with heavy mineral analysis magnetic fe oxide inclusions within framework clastic grains in sediments provide an indication of the provenance of the enclosing host silicate particles magnetic mineral inclusion characterisation is performed using a variety of magnetic properties that are related to magnetic mineral abundance magnetic grain size domain state oxidation state and magnetic grain interaction the magnetic methodology extends that based on a conventional set of environmental magnetic measurements using a variety of recent sediments transported from known rock sources within the uk and eire it is demonstrated using discriminant function analysis that the magnetic properties of the fe oxide inclusions provide a clear distinction of primary provenance this is both at the large scale in terms of various gneiss schist and granite sources and also at a smaller scale when considering subdivisions of these sources using case studies from the marine upper jurassic piper formation and non marine triassic sediments otter bank sandstone and foula sandstone formations it is shown that provenance differences clearly expressed by the results of heavy mineral analysis are also sensitively displayed by the magnetic mineral inclusion data the provenance classification is based around use of hierarchical cluster analysis and multidimensional scaling simple statistical tools are developed to determine a least noisy subset of magnetic parameters which are most suitable for stratigraphic provenance discrimination the triassic case study has indicated the magnetic mineral inclusion technique is most sensitive when using the sediment fractions larger than 150 m

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

the physical isolation and identification of carriers of geologically stable remanent magnetization paleomagnetic and rock magnetic microanalysis and electron microscopy paleomagnetic and rock magnetic microanalysis of fully oriented microspecimens provides a more direct understanding of the source and origin of geologically stable remanent magnetizations scanning transmission electron microscopy allows direct observation and characterization of magnetic phases we provide a brief history of micromagnetic research describe a functional microspecimen preparation method evaluate instrumentation and present results of three combined micromagnetic stem studies

modeling of cretaceous foreland basin parasequences utah with implications for timing of sevier thrusting computer modeling suggests that thrusting order can be discriminated on the basis of foreland basin geometry and parasequence stacking patterns the position of sequential thrust sheets relative to the basin affects the amount and distribution of subsidence source terrain elevation and distance from the shoreline and sediment transport into the basin for this study a forward numerical model was constructed to quantitatively simulate the interaction of geologic processes active in the foreland basin setting including thrusting and orogenic development lithospheric flexure orogenic denudation sediment transport across the coastal plain and nearshore deposition in order to isolate the effects of tectonics and sediment supply on nearshore deposition eustatic sea level is held constant model runs simulate two different thrusting sequences foreland stepping in sequence and hinterland stepping out of sequence when in sequence thrusting is modeled sediment is trapped in intra orogen basins because sediment is prevented from reaching the shoreline the basin is underfilled when out of sequence thrusting is modeled sediment is transported from the orogen to the shoreline episodes of thrusting result in subsidence and marine flooding surfaces and resulting accommodation space allows for parasequence development comparing model output to stratigraphy observed in eastern utah suggests that late cretaceous out of sequence uplift and thrust loading resulted in aggradational and progradational parasequence stacking in the campanian blackhawk formation this result supports the theory that the pavant thrust system was active prior to deposition of the blackhawk formation and that the location of thrust shortening shifted toward the hinterland prior to or during blackhawk formation deposition

depositional record of an avulsive fluvial system controlled by peat compaction neogene most basin czech republic this paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply at a range of spatial and temporal scales as documented by extensive exposures in an open cast mine in the most basin part of the oligo miocene oh e rift eger graben basin system in the czech republic the most basin is characterized by the occurrence of a number of phenomena resulting from syn and post depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness the studied strata are interpreted as deposits of an avulsive mixed load fluvial system the large scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick representing a record of long term evolution of a clastic floodplain bordered by accumulating peat within each of the packages several small scale channel belts were documented individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit two main mutually linked processes controlled the evolution of the studied fluvial system i syndepositional compaction of the underlying peat and ii avulsions of the channels away from the original floodplain resulting in formation of a new floodplain the processes which caused the channels of the hrabak fluvial system to reach the avulsion threshold were i decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope and ii cross floodplain tilting of the channel belt caused by differential compaction of underlying organic rich substratum

interpretation of magnetic anomalies due to dikes the complex gradient method a method to interpret the magnetic anomaly due to a dipping dike using the resultant of the horizontal and vertical gradients of the anomaly is suggested the resultant of both the gradients is a vector quantity and is defined as the complex gradient a few characteristic points defined on the amplitude and phase plots of the complex gradient are used to solve for the parameters of the dike for a dike uniformly magnetized in the earth s magnetic field the amplitude plot is independent of f the index parameter which depends upon the strike and dip of the dike and the magnetic inclination of the area the phase plot of the complex gradient is an antisymmetric curve with an offset value equal to f for a dike whose half width is greater than its depth of burial two maxima at equal distances on either side of a minimum value appear on the amplitude plot for a dike whose half width is equal to or less than its depth of burial the amplitude plot is a bell shaped symmetric curve with its maximum appe

crustal uplift in the southern superior province canada revealed by paleomagnetism abstract a change in the polarity of magnetization with depth in the 2 45 ga matachewan dyke swarm is used to document vertical crustal movements that occurred at 1 9 2 3 ga along the kapuskasing structural zone a 500 km long fault zone that transects the archean superior province of canada at shallow crustal levels dykes have a primary magnetization dominantly of one polarity but at greater depths 20 km down a polarity change occurs associated with the growth of exsolved magnetite in feldspar due to slow crustal cooling after cessation of matachewan igneous activity regions of the dyke swarm with one dominant polarity are separated from those with opposite polarity by major faults using this polarity distribution and associated variations in the intensity of feldspar clouding and hydrous alteration maps of the southern superior province are produced that display regional crustal tilting on which are superimposed more local fault bounded blocks associated with the kapuskasing zone some of these blocks have been recognized for the first time as a result of this study the paleomagnetic work has also shown that the matachewan swarm is regionally distorted both within and north of the kapuskasing zone and originally had a more radial disposition this widespread distortion suggests that the lower crust was still relatively ductile at the time of deformation perhaps due to high heat flow associated with the waning stages of the matachewan mantle plume beneath

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

titanomagnetite granulometry and rockmagnetism of some plio pleistocene volcanic rocks particle size distribution and composition of iron titanium oxides have been measured in four samples of volcanic rock from the lesser antilles magnetic hysteresis properties are predicted for these samples on the basis of available data for synthetic analogues and the predictions are compared with laboratory measurements of the same parameters

secondary ferrimagnetic minerals in welsh soils a comparison of mineral magnetic detection methods and implications for mineral formation summary a range of low temperature and isothermal magnetic measurements are used to identify the secondary ferrimagnetic mineral sfm grain sizes in 10 representative soil samples from wales a comparison of percentage frequency dependent susceptibility fd percentage and low temperature remanence measurements shows that they are sensitive to different ranges of superparamagnetic sp grains the relative distributions of sp grains and stable single domain ssd grains are similar in nine of the samples typical distributions for soils dominated by sfms are 20 30 per cent ssd and 70 80 per cent sp multidomain md grains were not detected in the samples studied there is evidence that some soils contain significant numbers of ultrafine sp grains 0 010 m that are not detected by low temperature remanence measurements at 20 k and which will have the effect of depressing values of low field susceptibility lf and fd percentage a mixing model suggests that fd percentage may be used semi quantitatively to estimate the proportion of sp grains in a sample the positively skewed grain size distributions strongly suggest a mechanism of sfm formation that is driven by processes at the 10 8 m scale thus supporting weathering and fermentation as controlling processes rather than the degradation of ssd bacterial magnetosomes and primary minerals

fluvial response in a sequence stratigraphic framework example from the montserrat fan delta spain abstract exceptional exposure of the montserrat fan delta system eocene in northeastern spain provides an excellent framework to evaluate the alluvial response to sea level changes over two different time scales the alluvial system contains multiple fifth order cycles 104 yr and eight fourth order cycles 105 yr fifth order cycles are characterized by long distance shoreline migrations and occasionally by incised basal scour surfaces but not by changes in fluvial style lithofacies or channel stacking pattern fourth order cycles are composed of stacked fifth order cycles and have non erosional basal boundaries vertical sedimentation rates and channel stacking patterns change significantly within fourth order cycles the lower parts of these cycles referred to as the transgressive facies tract show overall shoreline transgression and associated alluvial deposits contain abundant overbank materials with isolated ribbon channel bodies during this time the supply of terrigenous material to the shoreline was reduced as indicated by sediment starvation offshore the overlying middle part of fourth order cycles the lower regressive facies tract differs only in that shoreline is overall regressive and there is increased clastic supply to the offshore in the upper part of hese cycles the upper regressive facies tract channel stacking geometries become denser and more sheet like shoreline regression is more pronounced and vertical aggradation rates are inferred to be reduced changes in the alluvial system during fourth order cycles are most pronounced adjacent to shoreline and die away upstream over just a few kilometers indicating that the base level signal decays away over the distance of a few backwater lengths channel flow depth slope higher frequency fifth order changes in relative sea level appear to produce the largest shoreline migrations but lower frequency fourth order changes have more impact on the channel stacking architecture of the alluvial systems observed changes in alluvial stacking pattern may be most commonly found in tectonically active rapidly subsiding foreland basins because of their back tilted geometry we propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea level rise progradation occurs as erosion rates in the mountain belt increase and rates of subsidence and relative sea level rise diminish changes in alluvial architecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream hence changes in channel stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal part of the basin and are not necessari y driven by eustatic sea level changes

transmission electron microscope and rock magnetic investigations of remanence carriers in a precambrian metadolerite a precambrian metadolerite dyke has two distinct types of remanence carriers those with medium high coercivities unblocking fields of 20 120 mt and those with low coercivities unblocking fields of 15 mt optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation exsolution intergrowths of magnetite and ilmenite all opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction the submicron inclusions in the plagioclase are titanomagnetites 0 x 0 14 with a size range between about 0 01 and 0 5 m and axial ratios between 1 equidimensional and about 0 3 many of these inclusions fall in the single domain field but some are probably pseudo single domain the large opaque grains contain almost pure magnetite and ilmenite and show no fine scale exsolution the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under lorentz electron microscopy there are also some primary ilmenites containing very fine exsolved haematite and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20 140 mt and blocking temperatures of 390 590 c and the large opaque grains acquire a remanence with unblocking fields of less than 15 mt but a wide range of blocking temperatures up to about 570 c it is concluded that the medium high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques the contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape

palaeomagnetism of precambrian dyke swarms in the north china shield the 1 8 ga lip event and crustal consolidation in late palaeoproterozoic times abstract the north china shield ncs is cut by a laterally extensive dyke swarm emplaced at 1 78 1 76 ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by 1 85 ga palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with ne to n declinations and rare opposite polarity dykes with ne shallow magnetic declination a1 d i 36 1 recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations a2 d i 8 2 in lower grade metamorphic terranes to the south contact tests indicate a primary cooling related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at 1 85 ga a temporal migration a2 a1 is implied by the higher crustal level and earlier acquisition of the former and the deeper source and later acquisition of the latter a third population of dyke magnetisations a3 d i 18 43 is distributed towards steeper inclinations and close to the mesozoic recent palaeofield these are either partial or complete overprints of a1 a2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra or are much younger dykes of mesozoic tertiary age a minority fourth later precambrian but presently undated dual polarity population has a magnetisation 11 dykes d i 108 7 with contact tests indicating a primary cooling related origin the 1 78 1 76 ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major large igneous province lip related events the new definition of a 1 83 1 76 ga apw swathe from the north china shield permits a comparison with other shields and yields a constraint to continental configurations during the late palaeoproterozoic a quasi integral reconstruction of palaeopangaea is tested here and supported by conformity of predominantly of uplift related palaeopoles from the 1 90 1 70 ga tectono thermal belts and from sw ne trending apw implied by the distribution of poles from the 1 80 ga igneous suites including the lip events this trend incorporates the a2 a1 migration and the granulite terrane cooling polar swathe from north china the reconstruction indicates that continental crust consolidated in palaeoproterozoic times by accretion of 2 3 1 7 ga orogenic belts around a hemispheric and crescent shape core already established by late archaean times the north china shield is interpreted to have bordered the western cratonic margin of the indian shield in a proximity supported by correlation of geological features and suggested by a number of previous workers the central orogenic zone of the north china shield characterised by tectono thermal activity prior to 1 85 ga was then contiguous with a comparable zone running through the centre of the indian shield and continuing into the capricorn belt of western australia the 1 78 1 76 ga dykes in north china continue into dyke swarms in the south india shield and may have been sourced in a plume related lip focussed near the continental margin in the xiong er aulacogen

biogenic and lithogenic magnetic minerals in atlantic and pacific deep sea sediments and their paleomagnetic significance the carrier of the natural magnetization of deep sea sediments was characterized by mineralogical electron microscopic and rock magnetic investigations magnetic single domain sd and pseudo single domain psd particles which are most important for the stable remanent magnetization were separated from the magnetic coarse fraction and concentrated as magnetic fine fraction the magnetic coarse fraction consists of lithogenic magnetite and titanomagnetite which often contains exsolution lamellae of ilmenite both minerals are partially maghematized and occur isolated in the sediment or embedded in rock particles in regionally different concentrations the magnetic fine fraction consists of lithogenic magnetite and titanomagnetite and biogenic magnetite magnetofossils fossil bacterial magnetosomes the latter generally maghematized

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

palaeomagnetism of some precambrian rocks in south east greenland abstract a palaeomagnetic pole is established at 25 1 n 273 9 e dp 10 6 dm 14 3 from the norite charnockite complex at angmagssalik emplaced at 1800 ma a somewhat older palaeomagnetic pole at 4 2 s 246 7 e dp 4 2 dm 8 3 is obtained from archaean gneisses close to the northern boundary of the nagssugtoqidian mobile belt reversals of magnetization are present here both magnetizations were imposed during slow cooling following the late nagssugtoqidian metamorphism in general the gneisses dyke amphibolites and granite of the nagssugtoqidian mobile belt are unstably magnetized their magnetization is attributable to the earth s present field and is often extremely weak a pseudotachylyte within the archaean gneisses has had a long cooling history a fragment of the remanence reflects the magnetization characteristic of the archaean gneisses whereas most of the magnetization corresponds to a palaeomagnetic pole near that of the angmagssalik complex the pseudotachylyte is much older than its magnetizations an apparent polar wander path is presented for greenland at ca 1750 ma based on the above results and data from west greenland

nonlinear thermoremanence acquisition and implications for paleointensity data abstract in paleointensity studies thermoremanence is generally regarded as a linear function of ambient magnetic field at low fields comparable to that of the present day earth we find pronounced nonlinearity at low fields for a class of materials with silicate hosted magnetite that otherwise perform well in paleointensity experiments we model this nonlinearity with narrow size ranges of large acicular single domain grains which are most likely in a vortex state i e nonuniformly magnetized sometimes labeled pseudosingle domain simple trm theory predicts that even certain single domain particles will also exhibit a nonlinear response saturating in fields as low as the earth s such behavior although likely to be rare may bias some paleointensity estimates the bias is especially pronounced when the laboratory field is higher than the ancient field fortunately the fundamental assumption that thermoremanence is proportional to applied field can and should be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

tectonic control on fluvial styles the balfour formation of the karoo basin south africa abstract the balfour formation represents a fully fluvial succession of late late permian earliest triassic age which accumulated in the foredeep of the karoo basin during the overfilled phase of the foreland system the lack of a coeval marine environment within the limits of the preserved karoo basin provides an opportunity to study the stratigraphic cyclicity developed during a time when accommodation was solely controlled by tectonics the balfour stratigraphy is composed of a succession of six third order fluvial depositional sequences separated by subaerial unconformities they formed in isolation from eustatic influences with a timing controlled by orogenic cycles of loading and unloading sediment accumulation took place during stages of flexural subsidence whereas the bounding surfaces are related to stages of isostatic uplift the vertical profile of all sequences displays an overall fining upward trend related to the gradual decrease in topographic slope during orogenic loading at the same time an upward change in fluvial styles can be observed within each sequence from initial higher to final lower energy systems the actual fluvial styles in each location depend on paleoslope gradients and the position of the stratigraphic section relative to the orogenic front proximal sequences show transitions from braided to meandering systems whereas more distal sequences show changes from sand bed to fine grained meandering systems the average duration of the balfour stratigraphic cycles was 0 66 my i e six cycles during 4 my no climatic fluctuations are recorded during this time with the long term climatic background represented by temperate to humid conditions

on balanced and unbalanced accommodation peat accumulation ratios in the cretaceous coals from gates formation western canada and their sequence stratigraphic significance coal composition was investigated by means of photometric and maceral analyses on closely spaced lithotype based strip samples over the full thickness of several paralic coal seams from the cretaceous gates formation of the western canadian sedimentary basin the aim of this investigation was to test various methods of identifying accommodation trends in coal and use them to refine sequence stratigraphic interpretation of continental sediments conventional sequence stratigraphy derives its subdivisions and significant surfaces from the records left by relative sea level oscillations these records either do not project into the continental realm or are difficult to recognise in clastic non marine sediments paralic coal seams have been selected to study this problem because they are not entirely removed from marine influence and compared with most inorganic deposits coal has stored a greater wealth of information that can be analysed at a higher level of resolution the study has led to the identification of five new surfaces with chronostratigraphic potential in the sequence stratigraphic analysis of non marine sediments two of these surfaces called paludification surface pas and terrestrialisation surface tes occur at the bases of the investigated coal seams while two other surfaces referred to as non marine flooding surface nfs and give up transgressive surface guts form the tops of the coal the fifth and probably most important new surface called the accommodation reversal surface ars is independent of any particular facies and may either coincide with some of the other surfaces or occur separately the proportion of detrital minerals has been used as the chief discriminator between different mire types and accommodation trends other useful indicators of mire type and peat dispersal have been the proportions of sporinite and inertodetrinite as well as some derived maceral and or mineral ratios e g the groundwater influence index and the tissue preservation index isometamorphic variations of telovitrinite reflectance and fluorescence as well as their coefficients of variation were also found to contribute to the identification of cyclic shifts between balanced and unbalanced accommodation peat accumulation ratios some of these cycles which are backed up by clastic stratigraphy appear to correspond to the development of shallowing upward and deepening upward parasequences superimposed high frequency low amplitude perturbations in the coal cycles relate to smaller scale accommodation cycles of sub parasequence level not always recognised in non marine strata these sub parasequence coal cycles do not always continue the shallowing upward trend typical of conventional parasequences several coals were found to contain stacks of small scale cycles with upward increasing accommodation signatures either in their lower or upper halves or over the whole seam section

avulsion the relative roles of autogenic and allogenic processes abstract avulsion the abandonment of all or part of a channel belt in favor of a new course is controlled by both autogenic intrabasinal and allogenic extrabasinal processes however their relative roles are not known well the rhine meuse delta is without doubt the best studied delta regarding holocene avulsion processes based on studies of this delta an overview is given of 1 allogenic and autogenic processes influencing different aspects of avulsion 2 the relative importance of these processes in time and space and 3 observations that can be used to differentiate autogenic and allogenic causes of avulsion the following avulsion parameters are quantified and their allogenic and or autogenic nature are discussed period of activity of channel belts period between beginning and end of overbank sedimentation by the river channel interavulsion period time period between successive avulsions of a channel belt avulsion frequency number of avulsions per time interval in a given area and avulsion location in the rhine meuse delta the period of activity of the holocene channel belts varies considerably but shows no significant trend over time whereas extrabasinal factors climate base level and tectonics changed this suggests that this parameter is mainly autogenic controlled the average interavulsion period increased from 8000 cal yr bp to 2800 cal yr bp and decreased since then significant fluctuations occurred on a shorter time scale a maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr bp this was a time when large channel belts gradually came into existence during this period the avulsion frequency also reached a maximum the long term increasing trend in interavulsion period is related to the decreasing rate of sea level rise the decreasing interavulsion period since 2800 cal yr bp can be explained by increased discharge and sedimentation resulting in an increase in the number of avulsions seven avulsion sequences as predicted by the mackey and bridge mackey s d bridge j s 1995 three dimensional model of alluvial stratigraphy theory and application journal of sedimentary research b65 1 pp 7 31 model may be recorded in the holocene dataset of the rhine meuse suggesting a periodicity of 500 600 yr each avulsion sequence coincides with a peak in the avulsion frequency the periodicity of 500 yr in the avulsion frequency is proposed to be autogenic however there is also a long term trend in the avulsion frequency that can be related to sea level rise and after 3000 14c yr bp to climate change and human influence avulsion locations are not randomly distributed over the delta and are related to the allogenic factors relative sea level rise local tectonics and changes in discharge and sediment load

an extraordinary peat forming community on the falkland islands most of beauchene island in the south atlantic is covered by tussac the tussock forming grass poa flabellata lam rasp which has produced a deep accumulation of exceptionally dense peat during 12 500 yr the basal peat is lignitic yet it is several hundred times too young to be a true lignite during an ecological survey of the island in december 19801 one of us r i l s sampled an 11 m high peat face the age against depth profile in the peat is consistent with a constant proportional rate of decay of 1 1 2 2 10 4 yr 1 and a constant rate of addition of dry matter to the peat of 430 720 g m 2 yr 1 this rate of decay is within the range recorded for peats in corresponding latitudes in the northern hemisphere but the rate of addition of dry matter is about 10 times as great this is not easy to accommodate within current hypotheses about peat formation an unusual combination of biological physical and chemical circumstances may be the cause as the island is difficult to visit and no more information can be obtained in the near future we now report these results incomplete though they are

magnetic properties of synthetic analogues of the altered olivines of igneous rocks summary a laboratory simulation has been made of the high temperature deuteric alteration of olivine such as occurs in slowly cooled basic intrusions three compositions in the forsterite fayalite solid solution series were synthesized by a three firing method using self buffering and controlled atmosphere to obtain material containing no detectable magnetite fe o as a contaminant 0 005 per cent the prepared olivines were oxidized between 850 and 1280 c at oxygen fugacities of between lo and lop5 atmospheres the composition and concentration of the multiphase oxidation products were studied by magnetic optical electron optical and x ray methods the magnetic constituent of the intergrowth was magnetite below about 1150 c the microstructure is similar to the dendritic intergrowths found for example in the altered olivine of the central layered intrusion of the isle of rhum above about 1150 c the form may be similar to symplectite intergrowths the magnetization process parameters coercive force etc vary systematically with temperature of oxidation to which evidently controls the microstructure of the magnetite the magnetite produced is magnetically hard samples altered at 900 c and 950 c having higher coercive force hc than any magnetite reported in the literature the room temperature properties appear to bridge the published trends found in grown crystal magnetites and crushed grains inferred effective grain sizes of the complex intergrowths range from 0 1 to 3 pm as to increases from 900 to 1200 c the temperature dependence of magnetization process parameters principally h measured between room temperature and the curie point are modelled in terms of i monodomain grains subject to thermal fluctuations ii monodomain grains not subject to thermal fluctuations and iii as multidomain grains model i yields plausible values for effective grain dimensions and length diameter ratios higher to producing larger more equidimensional particles model ii suggests that the relative importance of cubic and uniaxial anisotropies may be influenced by a microstructure dependent uniaxial particle particle interaction term model iii implies that if domain walls are pinned by both surface and volume defects magnetostriction plays a greater role in volume pinning than in surface pinning plausible values of demagnetizing factors appropriate to grains with few domains are produced by model iii

magma flow pattern of the 1 78 ga dyke swarm of the north china craton during the initial assembly of the supercontinent nuna columbia constraints from rock magnetic and anisotropy of magnetic susceptibility studies abstract the geodynamic origin of the 1 78 ga dyke swarm in the north china craton ncc is long debated it is considered either a post collision process or paleoplume related constraining its origin is essential for the reconstruction of the ncc position within the supercontinent nuna also referred to as columbia here we report on the rock magnetism and anisotropy of magnetic susceptibility ams of thirty three 1 78 ga dykes to constrain the magma center of the igneous event the samples were collected from the entire swarm across the ncc magnetite appears to be the dominant magnetic mineral in all the dykes occasionally with minor maghemite while the magnetite grain size within the dykes is generally increasing from south to north ams results of the dykes portray a magma flow related fabric over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow while a few have been formed by vertical intrusion these observations imply that the 1 78 ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the ncc during dyke propagation some localized magma subchambers may have been formed they could be responsible for the less common vertically intruded dykes and the ew trending dykes therefore a plume related tectonic setting for the 1 78 ga dyke swarm is more compelling with the eruption center along the margin of the ncc it may serve as an essential criterion to search for possible counterparts in other tectonic units within nuna columbia to evaluate the proximity of the ncc with them

sequential architecture in a fluvial succession sequence stratigraphy in the upper cretaceous mesaverde group prince canyon utah abstract five unconformity bounded sequences have been documented within a 1300 m thick succession of predominantly fluvial upper cretaceous and lower tertiary deposits in the price canyon area of the westernmost part of the book cliffs east central utah u s a this area experienced major tectonically induced changes in the paleogeography during deposition the earliest fluvial deposits in the study area were formed in a relatively simple foreland basin setting in front of the sevier orogenic belt and contain more marine influence than previously recorded following a gradual termination of thrusting sedimentation became increasingly controlled by uplift of the san rafael swell to the southeast our studies suggest that the fluvial systems in the upper part of the succession alternated bet een being confined within a valley open towards the north and being more directly eastward flowing on the basis of the observed sequences an idealized model of alluvial sequences whose internal architecture can be related to a fall rise fall cycle of the stratigraphic base level has been established upward or downward changes in the position of this base level dictate the creation and destruction of accommodation on the alluvial plain and therefore exert a direct control on the sequential architecture and sandstone body geometry of fluvial successions in our model the basal sequence boundary is overlain by an amalgamated fluvial sandstone sheet the sheet is succeeded by a fining upward more mudstone rich level with more isolated sandstone bodies and this level may culminate in a marine or lacustrine transgression the upper part of the sequence may show a coarsening upward trend heralding the next phase of base level fall and sequence boundary generation use of the suggested model has the potential to refine existing lithostratigraphic schemes and given the higher resolution and more detailed correlation may significantly improve paleogeographic reconstructions and aid in prediction of potentially hydrocarbon bearing reservoirs a revised and refined lithostratigraphy has been established on the basis of alluvial sequence analysis the lower half of the succession forms the youngest part of a major eastward prograding clastic tongue the mesaverde group and is campanian we divide this part into the blackhawk castlegate and price river formations the rest of the succession is maastrichtian to paleocene and is referred to the north horn formation this formation was deposited in an intermontane setting

mechanisms controlling the clustering of fluvial channels and the compensational stacking of cluster belts abstract analysis of sand body distribution reveals that fluvial channel sands in the upper cretaceous lower williams fork formation in the central piceance basin colorado usa are not randomly distributed but are predictable in their spatial and stratigraphic position sand bodies are organized in channel belt clusters that are compensationally stacked on different temporal scales while channel clustering occurred over a shorter period channel and channel belt scale 10s ky to 100s ky compensational stacking occurred over a much longer time scale channel cluster belts 400 ky the lower williams fork formation consists of three distinct intervals of clustered channel belts each 400 ft 122 m thick topped by a thin coal layer the primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase short lived changes in in basin accommodation caused by either changes in tectonic subsidence or high frequency eustatic changes due to milankovitch band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle coals at the end of each sub cluster span the entire study area but their overall thickness is greater in the inter cluster floodplain prone areas the resulting differential early coal compaction is greatest in the inter cluster areas and has a significant control on the subsequent cluster belt position overlying the coal channel sands overlying the coals are concentrated in the previously low net gross floodplain prone inter cluster areas resulting in compensationally stacked cluster belts on the longest time scale 1 to 1 5 my which encompasses the entire lower williams fork changes in channel geometries and sand body thickness seem to be controlled mainly by long term changes in eustasy and autogenic processes are of lesser importance

magnetic fabric and post orogenic uplift and cooling magnetisations in a precambrian granulite terrain the datong huai an region of the north china shield abstract the polymetamorphic terrain in the datong huai an region of the north china shield records two metamorphic m1 and m2 and associated deformational d1 d3 and d4 episodes in granulite facies the magnetic fabric is dominated by the d3 foliation imparted by emplacement of allochthonous cover rocks onto basement during compression and subsequent collapse and uplift magnetite is present in four generations as 1 discrete grains 2 exsolution lamellae in mafic minerals 3 corona rims formed during regional decompression and 4 a filling of microfractures it is the dominant ferromagnetic phase and carries a stable high blocking temperature remanence of dual polarity linked to magnetite grain sizes and shapes of generations 2 to 4 remanence was acquired during later stages of uplift and cooling after the m2 metamorphism and d4 deformation and during rapid decompression a ne to nw migration of the palaeofield direction during this time is identified palaeomagnetic directions are not constrained to lineation in the older magnetic anisotropy but lie close to the dominant d3 foliation although no satisfactory methods presently exist for correcting remanence for anisotropy qualitative considerations from petrology and comparisons between sites of contrasting ams fabric suggest that palaeodirections are not prominently deflected into the d3 plane the pole positions define a continuous swathe plotting between 50 e 20 s and 170 e 35 s summarised by group mean poles a1 51 e 20 s dp dm 8 15 a2 80 e 33 s dp dm 6 11 a3 106 e 48 s dp dm 7 13 and a4 164 e 40 s dp dm 11 21 they are assigned to the interval ca 2000 1840 ma and extend the apparent polar wander record for the north china shield backwards in time from the record derived from supracrustal successions

how plausible are high frequency sediment supply driven cycles in the stratigraphic record abstract this paper is an attempt to quantify the plausible time scales of clastic sediment supply variations at the entrance of sedimentary basins our approach is based on the sedimentary system concept which simplifies natural systems by dividing them into three zones of dominant processes the erosion the transfer and the sedimentation subsystems we examine recent results from geomorphology which show that frequent climate changes can induce high frequency sediment flux variations at the outlet of the source area we put forward the crucial role of the transfer subsystem which conveys sediment from the erosion zone to the basin by applying a diffusive model to a number of worldwide rivers we extend from large 1000 km to intermediate 300 km rivers the previous finding that the transfer subsystem acts as a buffer for short periods sediment pulses tens to hundreds of kiloyears this implies that high frequency stratigraphic cycles in clastic accumulations fed by large drainage systems are unlikely to reflect sediment supply cycles of tens to hundreds of thousands of years of periodicities

two phase stratigraphic model of foreland basin sequences application of flexural models to nonmarine foreland basin evolution indicates that two different stratigraphic styles of basin fill may develop over time basin subsidence is most rapid during times of thrust load emplacement associated sedimentation is coarse grained immediately adjacent to the thrust front and grades rapidly into fine grained deposits that cover most of the basin the distal part of the basin may also contain deposits derived from streams that flow from beyond the basin toward the thrust belt subsequent removal of the thrust load by erosion and other processes results in flexural rebound of the thrust belt and adjacent foreland basin during this postorogenic phase of adjustment a regional unconformity develops in the proximal part of the foreland basin proximal deposits along with thrust derived sediment are redeposited in the distal foreland basin and beyond this two phase model of foreland sedimentation predicts that coarsening upward sequences in the proximal and distal parts of the basin have reciprocal significance the proximal sequence represents thrust belt advance whereas the distal sequence represents thrust belt cessation

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

palaeomagnetism rock magnetism and magnetic fabrics in precambrian metamorphic terranes of the huabei shield china abstract this study has investigated magnetic remanence rock magnetism and anisotropy of magnetic susceptibility ams in granulite and amphibolite grade metamorphic terranes of the huabei shield between inner mongolia in the west and the bohai sea in the east rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers a widely recorded stable remanence was probably fixed by grain shape effects granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and ams fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite an important component recognised as a partial or complete remagnetisation by late mesozoic tertiary tectonic magmatic activity is present in basement at the southern margin of the outcrop miyun terrane and where extensive granite plutonism has occurred zhunhua terrane these components have directions corresponding to remanence in the yunmeng shan granite 119 114 ma d i 33 58 39 samples a 95 3 5 palaeopole at 201 e 64 n most remanence elsewhere was probably acquired during post tectonic uplift and cooling of the basement between 2200 and 1850 ma because palaeomagnetic directions are removed from the phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying jixian supergroup deposited at 1840 900 ma these latter magnetisations are considered reliable indicators of the palaeofield during late palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed palaeofield directions and poles attributed to the time of uplift related cooling are qian an terrane d i 215 71 a 95 9 17 samples pole at 99 e 10 n and north qianxi terrane d i 44 45 a 95 4 41 samples pole at 79 e 11 s in addition a more widely preserved shallow northerly component correlates with a nw e swathe of components recorded by uplift related cooling within the datong huan an granulite terrane in the west of the shield a preliminary palaeo mesoproterozoic apparent polar wander path for the huabei shield is defined from the palaeoproterozoic record in the metamorphic basement rocks and the meso neoproterozoic record in the overlying jixian supergroup it incorporates a loop between 2200 and 1850 ma and exhibits a general east to west trend in subsequent times

magnetofossil dissolution in a palaeomagnetically unstable deep sea sediment magnetofossils1 the fossil remains of bacterial magneto somes2 are found in various deep sea sediments and have been linked to the preservation of stable natural remanent magnetization in many of them1 3 5 they have also been extracted and identified from lithified carbonates of jurassic5 and precambrian6 age showing that magnetotactic bacteria have been a sedimentary source of fine grained magnetite for much of geological time some clay rich deep sea sediments however do not record a stable remanent magnetization for unknown reasons here we report results from a high resolution transmission electron microscope study on samples of this type collected by the deep sea drilling project dsdp the material contains a complex mixture of single domain magnetic minerals well preserved magnetofossils show the same crystal structures as those found in magnetosomes from recent bacteria7 10 whereas others in these same preparations display a wide range of dissolution corrosion and aggregation effects rock magnetic measurements are consistent with the presence of these alterations in many dsdp sediments including those that preserve reliable palaeomagnetic directions as we were unable to fina authigenic magnetic minerals in our sample and as the magnetic fraction is dominated by well preserved magnetofossils we suggest that the poor preservation of the magnetization is a result of diagenetic interactions between the magnetofossils and the clay minerals in the matrix

factors controlling peat compaction in alluvial floodplains a case study in the cold temperate cumberland marshes canada abstract subsidence due to peat compaction may have important implications in alluvial floodplains because it leads to wetland loss land inundation and damage to buildings and infrastructure furthermore it potentially influences spatial and temporal river sedimentation patterns and hence the evolution of alluvial floodplains as the vast majority of peatlands occur in cold temperate regions alluvial plains situated in such areas are most susceptible to these implications to determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction induced subsidence it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction for this we carried out field work in the cumberland marshes east central saskatchewan canada guided by the following two questions 1 how much peat compaction has occurred over the past decades to centuries and 2 which factors control the amount and rate of peat compaction at the same timescale to address these questions we used methods involving construction of stratigraphic cross sections organic matter content and bulk density measurements and radiocarbon dating a new sampling device was devised to sample uncompacted peat in the field results show that peat layers have compacted up to 43 within a few centuries with compaction rates of up to 6 08 mm yr the dominant factors influencing peat compaction are 1 organic matter content 2 stress imposed on a peat layer and to a lesser extent 3 plant species composition in an alluvial setting crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales 100 102 years sheet like splay deposits initially compact underlying peat uniformly whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates

imaging and modelling the subsurface structure of volcanic calderas with high resolution aeromagnetic data at vulcano aeolian islands italy in this paper we present a magnetic model of the subsurface structure of vulcano island based on high resolution aeromagnetic data three profiles across the most intense magnetic anomalies over the piano and fossa calderas were selected for the magnetic modelling which was constrained by structural and volcanological data previous geophysical models paleomagnetic data and borehole stratigraphy obtained from two deep wells the interpretation of the magnetic sources represents a significant contribution to the understanding of the piano and fossa calderas underlying structure providing us with evidence of the lateral discontinuity between them at depth we propose that the positive magnetic anomalies in the piano caldera area are caused by a the remnants of an early submarine volcano b an outcropping dyke swarm related to the feeding system of the primordial vulcano phase beneath mt saraceno and c the presence of a non outcropping dyke system intruded along a ne sw oriented intra caldera fault beneath the eastern part of the piano caldera offshore to the west the magnetic anomaly map suggests the presence of a submarine volcanic structure not revealed by bathymetric data which could represent the eruptive centre the presence of which has been indirectly deduced from the outcrop of eastern dipping lavas on the western seashore magnetic modelling of the fossa caldera points to the presence of a highly magnetized cone like body inside the fossa cone centred beneath the oldest crater rims we interpret this body as a pile of tephritic lavas emplaced in an early phase of activity of the fossa cone suggesting that the volume of mafic lavas that erupted at the beginning of the construction of the fossa edifice was more significant than has previously been deduced furthermore the presence of a magnetized body inside the fossa cone implies that high temperatures are contained in very limited spaces do not affect its bulk inner structure and are restricted to fumarolic conduits and vents in addition structures beneath the western and northern part of the fossa caldera are revealed to have null or low magnetization which can be ascribed to the presence of pyroclasts and hyaloclastites in this area as well as to a large volume of hydrothermally altered materials this suggests that the hydrothermal system with a very limited extension at present affected a larger area in the past especially beneath the western part of the caldera

uplift and cooling magnetisation record in the bamble and telemark terranes sveconorwegian orogenic belt se norway and the grenville sveconorwegian loop article i nfo the 1100 ma sveconorwegian orogenic belt comprises allochthonous terranes juxtaposed by major fault zones and emplaced against and onto the south western margin of the fennoscandian shield to resolve the magnetic signature acquired during post orogenic uplift and cooling and evaluate wider correlations with the contemporaneous grenville belt of north america this study reports a regional palaeomagnetic study on a range of rock types from sectors of the medium high metamorphic grade bamble terrane 48 sites and 390 cores and the adjoining medium low grade telemark terrane 33 and 240 cores juxtaposed by an orogen parallel porsgrunn kristiansand fault zone with major vertical displacement magnetite and ilmeno hematites are magnetic carriers with the latter more significant in the higher metamorphic grades magnetic intensities are stronger in the higher grade terrane presumably due to the growth of metamorphic ferromagnets but are an order lower than values predicted for the lower continental crust and indicate that an additional mechanism is responsible for high magnetisations in deep crust anisotropy of magnetic susceptibility ams largely reflects the ne sw tectonic grain of the last stage of sveconorwegian ductile deformation the magnetisation record is filtered by excluding magnetisations possibly acquired during regional mesozoic dyke emplacement development of the permo carboniferous oslo rift and late proterozoic magmatism the remaining record is a dual polarity signature summarised by mean poles at 31 9 n 50 9 e n 191 components in the bamble terrane and at 34 2 n 58 9 e n 151 components in the telemark terrane however these means are non fisherian and embrace arcuate distributions of magnetic components acquired during protracted exhumation cooling of the orogen with the best defined parts

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

new interpretations on palaeomagnetic data from the nagssugtoqidian mobile belt in greenland abstract the principles governing directions of magnetisation in a metamorphic terrain which has experienced slow cooling during uplift are reiterated these principles are illustrated in areas affected by the nagssugtoqidian metamorphism in west and east greenland directions of magnetisation in the kang miut dykes in the nagssugtoqidian block and in the craton to the south are similar to each other the directions in the dykes are close to those in the surrounding gneisses implying magnetisation within a relatively short time their palaeomagnetic poles are close to those from nagssugtoqidian localities elsewhere in west and east greenland between site and within site directions in the dykes are asymmetrical which is attributed to apparent polar wandering a statistical method of plotting apparent polar wander paths is presented such paths from dykes in the archaean and nagssugtoqidian regions are similar from which a composite path is drawn thus establishing that the apparent polar wander path for greenland lies between 0 n 310 e and 22 n 260 e in the palaeoproterozoic at about 1750 ma upon cooling after the nagssugtoqidian metamorphism similar apparent polar wander paths have been drawn from other localities in west greenland numerous poles from east and west greenland fall close to this path and are thus of nagssugtoqidian age the mean nagssugtoqidian pole from eight representative studies lies at 20 3 n 278 3 e dp dm 4 8 6 4

fluvial responses to climate and sea level change a review and look forward summary fluvial landforms and deposits provide one of the most readily studied quaternary continental records and alluvial strata represent an important component in most ancient continental interior and continental margin successions moreover studies of the long term dynamics of fluvial systems and their responses to external or allogenic controls can play important roles in research concerning both global change and sequence stratigraphy as well as in studies of the dynamic interactions between tectonic activity and surface processes these themes were energized in the final decades of the twentieth century and may become increasingly important in the first decades of this millennium this review paper provides a historical perspective on the development of ideas in the fields of geomorphology quaternary geology vs sedimentary geology and then summarizes key processes that operate to produce alluvial stratigraphic records over time scales of 103 106 years of particular interest are changes in discharge regimes sediment supply and sediment storage en route from source terrains to sedimentary basins as well as changes in sea level and the concept of accommodation late quaternary stratigraphic records from the loire france mississippi usa colorado texas usa and rhine meuse the netherlands rivers are used to illustrate the influences of climate change on continental interior rivers as well as the influence of interacting climate and sea level change on continental margin systems the paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea level change at present empirical field based research on fluvial response to climate and sea level change lags behind a the global change community s understanding of the magnitude and frequency of climate and sea level change b the sequence stratigraphic community s desire to interpret climate and especially sea level change as forcing mechanisms and c the modelling community s ability to generate numerical and physical models of surface processes and their stratigraphic results a major challenge for the future is to catch up which will require the development of more detailed and sophisticated quaternary stratigraphic sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings there is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter 102 103 years and longer 104 106 years time scales than has commonly been the case to date as well as in larger river systems from source to sink studies of quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre quaternary rock record

an algorithm for determining grainsize spectra from plane of cut information an algorithm is developed for obtaining grainsize distribution by volume from observations of areas of particles exposed in a plane of cut the approach follows existing methods except that error estimates are included and a logarithmic scale of grainsize is adopted one tenth decade class intervals in grainsize are suggested arid there is no limit on the number of orders of magnitude which can be accommodated the scheme was devised to permit the inventory of magnetic minerals in rocks to be studied but it may be useful in other branches of petrology

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

magnetic models of crystalline terrane accounting for the effect of topography igneous rocks commonly have large magnetic susceptibilities so that high topographic relief in crystalline terrane can produce significant anomalies in aeromagnetic surveys topographic anomalies are particularly significant in relatively undeformed volcanic terrane because young volcanic rocks generally have large natural remanent magnetizations as well as large susceptibilities these anomalies commonly appear in aeromagnetic surveys as a complex pattern of high amplitude short wavelength magnetic features that tend to obscure anomalies caused by deeper geologic sources we have facilitated geologic interpretation of an aeromagnetic survey of the oregon cascade range by calculating the magnetic field caused by a three dimensional 3 d topographic model maps of the calculated field are compared with observed aeromagnetic data both visually and with a numerical technique that produces a contour map of correlation coefficients for the model these comparisons allow quick recognition of anomalies caused

sandstone body dimensions in a lower coastal plain depositional setting lower williams fork formation coal canyon piceance basin colorado this study addresses the field scale architecture and dimensions of fluvial deposits of the lower williams fork formation through analysis of outcrops in coal canyon piceance basin colorado the lower williams fork formation primarily consists of mud rock with numerous isolated lenticular to channel form sandstone bodies that were deposited by meandering river systems within a coastal plain setting field descriptions global positioning system traverses and a combination of high resolution aerial light detection and ranging data digital orthophotography and ground based photomosaics were used to map and document the abundance stratigraphic position and dimensions of single story and multistory channel bodies and crevasse splays the mean thickness and apparent width of the 688 measured sandstone bodies are 12 1 ft 3 7 m and 364 9 ft 111 2 m respectively single story sandstone bodies n 116 range in thickness from 3 9 to 29 9 ft 1 2 to 9 1 m and from 44 1 to 1699 8 ft 13 4 to 518 1 m in apparent width multistory sandstone bodies n 273 range in thickness from 5 0 to 47 1 ft 1 5 to 14 4 m and from 53 2 to 2791 1 ft 16 2 to 850 7 m in apparent width crevasse splays n 279 range in thickness from 0 5 to 15 0 ft 0 2 to 4 6 m and from 40 1 to 843 3 ft 12 2 to 257 0 m in apparent width these data show that most sandstone bodies are smaller than the distance between wells at 10 ac spacing 660 ft 201 m analyses of interwell sandstone body connectivity suggest that even at 10 ac spacing only half of the sandstone bodies are intersected and few are intersected by more than one well

soil magnetic susceptibility and its relationship with naturally occurring processes and soil attributes in pedosphere in a tropical environment abstract soil magnetic susceptibility has potential to be used as a pedoenvironmental indicator from which mineralogy pedogeochemical pedogeomorphological and pedogenic processes can be inferred it can be used in pedosphere studies as an auxiliary information for appropriate and sustainable soil use and management this research aimed to analyze how pedogenesis and geochemical processes affect the and some of its attributes as well as its potential use in discriminating soil great groups following the digital soil mapping approach the study area is located in sao paulo state brazil soil samples were collected for physical chemical analysis from 79 locations 0 20 cm depth at these sites magnetic susceptibility was measured with a portable field instrument and analyzed in terms of geology relief and soil class the results showed that geology strongly affects mainly in diabase derived soils followed by metamorphosed siltstone and siltstone in fluvial sediments the exhibits different behaviors due to different sediments deposited by the capivari river in less evolved soils such as cambisols lithology is a more important contributor to than pedogenesis in more evolved soils pedogenesis increases whereas argilluviation ferralitization reduces it the values did not decrease significantly or even increase downslope due to the presence of diabase on the lower parts differences in where observed between diabase bedrock located in different parts of the study area indicating more of an influence by geomorphic processes rather than lithology with respect to soil attributes positive correlations between and base saturation cation exchange capacity organic matter and iron and clay content were found whereas a negative correlation was found between and sand content the correlates with changes in lithology and soil class demonstrating its application as a potential tool for the discrimination of soil great groups and digital soil mapping

aeromagnetic data provide new insights on the volcanism and tectonics of vulcano island and offshore areas southern tyrrhenian sea italy 1 the active vulcano island southern tyrrhenian sea represents the southernmost portion of a nw se elongated volcanic ridge that includes also lipari and salina islands the ridge is affected by a regional nw se to n s striking fault system the elaboration and analysis of data from three high resolution aeromagnetic surveys carried out between 1999 and 2004 on vulcano and offshore allow us to recognize high intensity magnetic anomalies related to volcanic centers conduits or shallow intrusions previously unreported offshore submarine vents have been also recognized some of them may correspond with source areas of outcropping exotic pyroclastics on vulcano the spatial analysis of the recognized magnetic anomalies and volcanic structures shows that they are preferably aligned along the strikes of the main regional faults that affect the volcanic ridge submarine volcanic conduits revealed by the aeromagnetic survey might represent potential sources for future submarine effusive or explosive activity

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

global correlation of cenomanian upper cretaceous sequences evidence for milankovitch control on sea level we have investigated the sequence stratigraphy of two widely separated marine cenomanian successions in southeast india and northwest europe and used high resolution ammonite biostratigraphy to demonstrate that sea level changes are globally synchronous and therefore must be eustatically controlled sequence scale sea level changes in the cenomanian were driven by the long eccentricity cycle 400 k y in the milankovitch band we hypothesize that during pre quaternary time the third order sequences of vail and haq are essentially a sediment response to sea level changes driven by the 400 k y cycle construction of a relative sea level curve for the marginal marine succession in india demonstrates that the short term sea level changes are rapid 10 100 m m y and have a magnitude of 2 20 m glacioeustasy is a possible but unproven driving mechanism

fourth order nonmarine to marine sequences middle castlegate formation book cliffs utah the lower part of the campanian castlegate formation one of the best known fluvial sheet sands of the western interior seaway combines with an overlying finer grained unit middle member of the castlegate formation to form an unconformity bounded third order stratigraphic sequence 3 m y duration this sequence contains a mappable muddy zone along the western book cliffs of utah now mapped as contiguous with the open marine parts of the succession buck tongue sego sandstone anchor mine tongue to the east the correlation within the middle castlegate has been refined and the nature of the link between tidally influenced fluvial strata in the west and marine strata in the east has been remapped five high frequency stratigraphic sequences all within the upper part of the larger third order sequence forming a thickness to 160 m have been mapped in the price area southeastward into the time equivalent marine succession near green river individual sequences probably 0 5 m y duration reflecting transgressive to regressive estuary infilling have an internal architecture in which fluvial and tidally influenced distributary channel belts pass up into muddy central basin sedimentary units to bayhead delta deposits and in some places up to more fluvial channel belts the key to the mapping of individual sequences is recognition of the muddy brackish water interval developed during maximum transgression of the time equivalent shoreline the two most proximal marine influenced sequences 2 and 3 are probably broadly time equivalent with the buck and anchor mine tongues to the east the analysis here contrary to recent suggestions shows that the marine sequences in the eastern book cliffs can be traced far into the time equivalent western fluvio estuarine succession

plagioclase hosted fe ti oxide micro inclusions in an oceanic gabbro plagiogranite association from the mid atlantic ridge at 13 34 n