flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

paleomagnetism and 40ar 39ar geochronology of the high grade metamorphic rocks of the jequie block sao francisco craton atlantica ur and beyond abstract paleomagnetic magnetic anisotropy data and 40 ar 39 ar ages are presented for high grade metamorphic rocks of the jequie block sao francisco craton jequie charnockites and enderbites gneisses from the eastern border of the block present northern and steep downward directions carried by ti poor titanomagnetite with high unblocking temperatures 550 600 c a mean direction for 12 sites of enderbites from the eastern sector yielded a magnetic component a d m 47 0 i m 75 7 95 6 3 k 48 8 with a corresponding paleomagnetic pole at 339 6 e 5 4 n a 95 11 2 sites sampled on other metamorphic rocks including granulite facies tonalites and dacites yield different magnetic components anisotropy of magnetic susceptibility measured for all sampling sites shows a high degree of anisotropy p 1 121 1 881 with ne or nw trending magnetic lineations and vertical magnetic foliations these data were used to correct the mean site directions for all components while the other components presented larger scatter after anisotropy correction the component a shows a slightly tighter clustering of magnetic directions d m 61 2 i m 76 5 95 5 4 k 66 2 giving a new anisotropy corrected paleomagnetic pole at 342 1 e 0 5 n a 95 9 6 40 ar 39 ar plateau ages of 2035 4 ma hornblende and 1876 4 ma biotite obtained for one of the samples with component a imply a very low cooling rate of 1 4 c ma for these rocks based on these ages and corrected unblocking temperatures of the magnetic component a we argue that the characteristic magnetization of the jequie metamorphic rocks was acquired by high temperature thermo chemical processes during regional cooling of the adjacent itabuna salvador curaca belt between 2089 and 1985 ma pole age likely at 1 99 ga comparison of this pole with available paleomagnetic poles from south america and africa suggests that neither atlantica nor ur have ever existed and dispersed continental fragments dominated the paleogeography at ca 2 0 ga ago

flow patterns of magma in dikes makhtesh ramon israel directions of magma flow were measured in a system of radial dikes in makhtesh ramon israel the flow directions were determined from field observations of segments fingers grooves and groove molds of the dikes the study indicates that the mean axis of magma flow is subhorizontal toward the north in agreement with the direction of divergence of the radial dike system two modes of flow were observed 1 regular bedding parallel flow in the well stratified rock units and 2 irregular meandering flow in the massive rock units it is suggested that corrugated dike walls in well stratified host rocks cause magma channelization and random or self generated restrictions in massive host rocks cause the apparent meanders furthermore the major lithologic boundaries in the host units strongly affect segmentation of the dikes

detection of astronomical cycles in the stratigraphic record by frequency modulation fm analysis abstract frequency modulation fm analysis of a sequence of basinal limestone shale cycles from the upper pliensbachian domaro limestone lombardy pre alps northern italy reveals modulations with multiple low frequencies that closely resemble those of the theoretical equatorial orbitally forced precession dominated insolation there is evidence for a systematic shift in these modulation frequencies that is consistent with a shorter jurassic length of day these results were used to estimate the incremental time scale along the domaro succession revealing a dual frequency signal comparable to the precession separate analysis of the limestones and shales suggests that the limestones were deposited during times of decreasing or low northern spring equatorial insolation nsei and the shales during times of increasing or high nsei we discuss two end member models that could link precession forced insolation to the cyclic basinal sedimentation the ambiguity of the source of carbonate influx into this early jurassic basinal depocenter led us to consider 1 an allodapic mud model in which precession forced sea level oscillations instituted a cyclic production of lime mud in shallow marine environments and subsequent transport into the adjacent basinal environments and alternatively 2 a pelagic ooze model in which precession forced insolation directly forced pelagic productivity and influx into the basin

lower triassic peritidal cyclic limestone an example of anachronistic carbonate facies from the great bank of guizhou nanpanjiang basin guizhou province south china abstract most shallow marine sections described from the lower triassic formed in mixed carbonate siliciclastic systems shallow marine limestones from an isolated carbonate platform the great bank of guizhou in the nanpanjiang basin south china provide a unique opportunity to examine a pure peritidal carbonate depositional system the platform had a low relief bank profile in the early triassic with oolite shoals at the margin shallow subtidal and peritidal deposits in the interior and pelagics debris flow deposits and turbidites on gentle basin margin slopes interior strata are 265 m thick beginning in the lowermost triassic with renalcis biostromes followed by lime mudstone oolite and cyclic peritidal limestone the cyclic limestone is olenekian in age 164 m thick and contains up to 83 cycles parasequences parasequences are 0 2 7 4 m thick and typically shallow upward from subtidal oolite grainstone and skeletal packstone facies followed by renalcis reef mounds or biostromes and capped by intertidal flaser bedded ribbon rock the skeletal packstone and reef mounds contain a low diversity biota of cyanobacteria echinoderms bivalves gastropods lingulid brachiopods spirorbids and ostracodes ribbon rock contains alternating lime mud and fine peloidal packstone and grainstone laminae scour surfaces ripple cross lamination lime mud drapes and minor prism cracks parasequence stacking patterns exhibit gradual increase and decrease of parasequence thickness defining a third order sequence boundary and several 4th order parasequence sets that correlate between interior sections parasequence stacking patterns similar to those of lower palaeozoic green house sequences and different from ice house sequences the lower triassic platform interior parasequences represent an anachronistic facies in that they are strikingly similar to the common renalcis mound ribbon rock parasequences of the lower palaeozoic and different from other permo triassic parasequences e g capitan shelf parasequences or loferites facies similarities such as renalcis mounds and flaser bedded intertidal ribbon rock reflect anomalous oceanic conditions resulting in low biodiversity and low intensity of bioturbation after the end permian extinction similarities in parasequence stacking patterns reflect low amplitude high frequency sea level fluctuations resulting from green house conditions common to the early palaeozoic and early triassic

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

pennsylvanian time scales and cycle periods geochronological results from central europe indicate that the duration of pennsylvanian time is only 19 m y compared to the harland et al and palmer estimates of 34 m y prior calculations of pennsylvanian cycle periods from the midcontinent of north america suggesting a fit with milankovitch orbital parameters may well be in errors as a consequence other mechanisms for possible eustatic sea level changes represented in those cycles are needed calculation of cycle periods of 100 ka or less lack precision in stratigraphic intervals representing ages characterized by error margins of millions of years thus cycle periods may be less reliable as an indicator of global process than previously considered particularly in rocks of paleozoic and early and middle mesozoic age

cyclicity and paleoenvironmental dynamics rocknest platform northwest canada the superbly exposed rocknest formation northwest territories canada contains 140 to 160 shallowing upward cycles 1 to 15 m thick that contain distinctive marker beds and can be correlated for 200 km parallel to strike and 120 km perpendicular to strike cycles are grouped according to cycle base lithology which reflects paleogeographic position on the platform shale based cycles of the distal inner shelf pass westward into dolosiltite based cycles then into tufa based cycles of the shoal complex and then into grainstone based cycles of the outer shelf cycles are bounded by erosional surfaces that become less pronounced away from the shoal complex cycles formed in response to small asymmetric rapid rise slow fall eustatic oscillations in sea level 10 m during sea level rise rapid transgression and submergence of tidal flats occurred along the eastern margin of the shoal complex followed by eastward expansion of the shoal complex and slow progradation of tidal flats over lagoonal facies during sea level fall a glacio eustatic model for cyclicity accounts for the data favoring asymmetric eustatic sea level oscillations subaerial exposure and vadose diagenesis of platform during sea level falls arrested progradation and incomplete shoaling of many cycles and cycle periods of 20 000 to 100 000 yr 18 000 to 30 000 yr for rocknest inner shelf slopes ranged from 8 cm km average at high sea level stand to 2 cm km average at low sea level stand tidal flat progradation rates were 7 to 9 km 1 000 yr interaction of inner shelf slope sea level fall rate and sedimentation rate generates complete condensed or expanded cycle profiles one dimensional computer modeling reveals that sedimentation rates of shoal complex facies were 250 cm 1 000 yr that lag times in sedimentation following transgression may have ranged from zero on the shoal complex to 2 000 yr on the distal inner shelf and that subtidal sedimentation rates may have ranged from 80 cm 1 000 yr for the proximal inner shelf to 20 cm 1 000 yr for the distal inner shelf

the pennsylvanian permian transition in the low latitude carbonate record and the onset of major gondwanan glaciation abstract recent studies suggest a marked expansion of glacial ice across much of gondwana beginning in the earliest permian because expansion of glacial ice results in a lowering of sea level the imprint of ice expansion should be evident worldwide as significant exposure event hiatuses or other evidence for sea level drop at or near the pennsylvanian permian boundary this literature review investigates the signature of an early permian expansion of gondwanan ice through examination of stratigraphic records from eight carbonate dominated palaeotropical regions across pangaea tropical carbonate environments are used because most form in tectonically quiescent regions and are sensitive indicators of eustatic change correlation between stratigraphic sections is achieved using the most current biostratigraphic and absolute time constraints available all studied sections show a sequence boundary or basinward shift in facies at or near the pennsylvanian permian boundary supporting the hypothesis of a significant expansion of glacial ice and global eustatic lowstand beginning in the early permian by contrast a series of mid sakmarian kungurian transgression events in the palaeotropics are interpreted to reflect the asynchronous deglaciation of gondwana the stratigraphic framework developed herein will allow for better correlations among stratigraphic records from gondwana and northern pangaea which will ultimately improve the understanding of how carbonate systems respond to global icehouse conditions such as during the late palaeozoic

sedimentary evidence of early late permian mid latitude continental climate variability southern bogda mountains nw china abstract a preliminary early late permian mid latitude continental climatic record in nw china was interpreted mainly from sedimentary climate indicators and type and stacking pattern of depositional systems and cycles in a 1178 m fluvial lacustrine section depositional systems analysis delineated five types of primary fluvial and lacustrine depositional cycles which were grouped into three high intermediate and low orders semi arid subhumid and humid climate types in terms of relative precipitation evaporation ratio were interpreted and climate variability was identified at sub cycle and high intermediate and low order cycle scales early kungurian climate fluctuated between subhumid to humid and middle late kungurian climate shifted gradually from subhumid semi arid to semi arid roadian climate fluctuated widely from humid to semi arid with strong precipitation seasonality at sub cycle and high order cycle scales wordian climate was dominantly humid with short subhumid intervals and ended with a peak semi arid condition capitanian climate was mainly subhumid to humid as the start of a long interval of dominantly humid strongly wet dry conditions into the wuchiapingian time the late kungurian wordian semi arid condition is incompatible with modern mid latitude east coast humid climate suggesting different mechanisms operating in pangea and panthalassa the highly variable roadian wordian climate may have started terrestrial mass extinction that climaxed at the end of permian

testing periodicity of depositional cyclicity cisco group virgilian and wolfcampian texas abstract periodicities of 15 cyclic sections of the cisco group are tested through gamma and spectral analyses the sections are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the eastern shelf north central texas in late pennsylvanian and early permian time gamma analysis generates facies dependent thickness time conversion factors s to construct corrected time series which are more realistic than the uncorrected series obtained by proportional conversion positive and stable s and improved resolution of the corrected spectra indicate successful tuning on the cisco spectra a short eccentricity peak 95 131 ky is the most prominent and persistent long eccentricity 413 ky and obliquity 34 and 43 ky peaks are persistent and precessional index peaks 17 and 21 ky are less persistent a persistent peak of period 50 80 ky may be a combination tone between obliquity and precessional index cycles non milankovitch peaks are sporadic the validity of gamma and spectral results is indicated by consistent estimated ages among the cisco sections and reasonable effective sedimentation rates calculated on the basis of milankovitch spectral calibrations the results combined with geologic evidence suggest that the cisco cyclicity was controlled dominantly by milankovitch climatic forcing

hunting for paleoclimatic periodicities in a geologic time series with an uncertain time scale an 8 m section of a mid cretaceous albian deep sea sediment core from piobbico italy contains 2 cycles m compositional oscillations as postdepositional chemical migration in the core is weak the relative amount of carbonate is a primary indicator of cretaceous paleoclimate variability we report results of spectral analysis of two series caco3 wt measurements taken at roughly 2 cm intervals and a more densely sampled photodensitometer record of light dark variation the uncertainty in the age depth relation i e unknown fluctuations in sedimentation rate hampers a detailed assessment of the suggested eccentricity correlation we fit for long term time scale greater than 75 kyr fluctuations in sedimentation rate by tracking the local frequency modulation of the dominant harmonic in the data series assuming the true input signal to be monochromatic we estimate uncertainty in the observed local frequency by means of a statistical jackknife we fit a smoothed cubic spline to the set of discrete frequency modulation estimates specifying a 2 n misfit criterion where n is the number of frequency estimates from this analysis we infer fluctuations of a factor of two in the sedimentation rate which occur predominately on a time scale of 400 kyr we generate tuned series using several candidate age depth relations from the carbonate and densitometer series the tuned carbonate series returns greater correlation with the modern milankovitch periodicities the 2 cycles m energy resolves principally into two harmonic components whose frequency ratio corresponds to an observational accuracy of 1 2 to that of the 97 0 and 127 6 kyr eccentricity oscillations at frequencies lower than 2 cycles m we find harmonic oscillations whose phase correlates with the beating envelope of the 2 cycles m energy in a manner consistent with the modern milankovitch insolation we also observe a sinusoidal oscillation of lower amplitude with period scaled by those of the presumed eccentricity oscillations of 39 2 1 1 kyr which agrees with the period of the modern obliquity cycle after correction for the effects of tidal friction oscillations corresponding to milankovitch precessional frequencies are not established owing possibly to low signal level and unknown short term deposition fluctuations the tuned densitometer series recovers few new periodic features although its inferred age depth relation correlates well with that of the carbonate series we calibrate the depth variable downcore to the orbital cycles to estimate r 5 0 0 15 m m y as the effective sedimentation rate

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

astronomical theory of the pleistocene ice ages a brief historical review abstract this paper traces the evolution of the astronomical theory of the pleistocene ice ages from its origin in the work of adhemar and croll through its quantitative transformation in the hands of milankovitch to its current renaissance emphasis is given to the interplay between climatic theory and geologic observation from 1941 when milankovitch published his final version of the theory until the early 1970s most attempts to test the theory empirically were carried out in the time domain by comparing the geologic record of climate with predicted climate curves these attempts ran into serious difficulties particularly the inaccuracy of the geologic time scale and the ambiguity of climate predictions early attempts to evaluate the theory with simple radiation balance models appeared to indicate that the climatic response to secular changes in the orbit would be too small to be observed geologically but by the mid 1970s the advent of an improved time scale for pleistocene marine deposits and the availability of long closely sampled records made it possible to arrange generalized but unambiguous tests of the theory in the frequency domain by comparing orbital and climatic spectra these results decisively confirmed the idea that orbital variations significantly influenced the earth s climate during the pleistocene but they did not tell us how this influence was exercised the emphasis of current work has therefore shifted to the identification and quantitative modeling of physical mechanisms by which the climate system responds so dramatically to the predicted rather subtle changes in radiative boundary conditions

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

the 1420 ma indiavai mafic intrusion sw amazonian craton paleomagnetic results and implications for the columbia supercontinent abstract the configuration and the timing of assembly and break up of columbia are still matter of debate in order to improve our knowledge about the mesoproterozoic evolution of columbia a paleomagnetic study was carried out on the 1420 ma indiavai mafic intrusive rocks that crosscut the polycyclic proterozoic basement of the sw amazonian craton in southwestern mato grosso state brazil alternating field and thermal demagnetization revealed south southwest chrm directions with downward inclinations for sixteen analyzed sites these directions are probably carried by sd psd magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests including thermomagnetic data hysteresis data and the progressive acquisition of isothermal remanent magnetization different stable magnetization components isolated in host rocks from the basement 10 km nw away to the indiavai intrusion further support the primary origin of the chrm a mean of the site mean directions was calculated at dm 209 8 im 50 7 95 8 0 k 22 1 which yielded a paleomagnetic pole located at 249 7 e 57 0 s a95 8 6 the similarity of this pole with the recently published 1420 ma pole from the nova guarita dykes in northern mato grosso state suggests a similar tectonic framework for these two sites located 600 km apart implying the bulk rigidity of the rondonian san ignacio crust at that time furthermore these data provide new insights on the tectonic significance of the 1100 1000 ma nova brasil ndia belt a major ew feature that cuts across the basement rocks of this province which can now be interpreted as intracratonic in contrast to previous interpretation from a global perspective a new mesoproterozoic paleogeography of columbia has been proposed based on comparison of these 1420 ma poles and a 1780 ma pole from amazonia with other paleomagnetic poles of similar age from baltica and laurentia a reconstruction in agreement with geological correlations

evidence of astronomical forcing of the earth s climate in cretaceous and cambrian times abstract we have completed analyses of three sections of shallow marine sedimentary cycles that were deposited at about 80 ma and between about 510 ma and 530 ma in different parts of north america in spite of the antiquity of these strata and the differences in the age location and cycle types power spectra in all of the records have distinct spectral peaks with ratios that are consistent with forcing by eccentricity obliquity and climatic precession for two examples utah 530 ma and the southern canadian rockies 510 ma the spectra reveal prominent peaks corresponding to obliquity with a period closer to that estimated for early paleozoic time than for the modern these are the first results supporting recent calculations of changes in astronomical periods over geological time we were able to extract these intriguing results from rocks as old and as poorly dated as these only after applying a new method the gamma method which we have developed for scaling time in cyclic strata the results are evidence that astronomical forcing has left a robust signal in the geological record and that the astronomical periods observed in the geological data are consistent with recent calculations of the shifts in those periods over phanerozoic times

distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation cisco group virgilian and wolfcampian north central texas meter scale transgressive regressive cycles of the subsurface cisco group are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the eastern shelf of the midland basin during late pennsylvanian and early permian time five cycle types are characterized by thickness magnitude order and principal lithofacies cycle magnitude is defined as the maximum facies shift in a cycle indicating extent of shoreline migration cisco cycles belong to three orders minor intermediate and major and they are superimposed and form a stratigraphic hierarchy each order of cycles has a distinct range of thickness and possibly duration a cycle is also divided into a lower sand poor interval during which coarse siliciclastic supply at the depositional site was diminishing and an upper sand rich interval during which coarse siliciclastic supply was high regional thickness and lithofacies variations of sand rich intervals indicate that progradational infilling at a depositional site lagged marine regression suggesting a delay in sediment supply from the upland source relative to the time of base level fall regional systematic variations in cycle abundance continuity and characteristics along depositional dip and strike record the interplay among regional topography pattern of siliciclastic supply and shelf subsidence which controlled distribution of depocenters and bypass zones and thus stratigraphic completeness and resolution regional persistence of cycles suggests a eustatic control on regional ordered transgressive regressive events in contrast local variations of cycle characters suggest controls by local topography and depositional dynamics which determined depositional loci differential compaction and erosion a predominantly autocyclic cisco record in the upper platform does not imply the absence of allogenic processes an allocyclic cisco record in the lower platform contains abundant autocyclic imprints because allogenic controls on cyclic sedimentation were accomplished through local autogenic processes distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation is an important step in establishing a high resolution meter scale chronostratigraphy of any sedimentary record 1

the form distribution and anisotropy of magnetic susceptibility of jurassic dykes in h u sverdrupfjella dronning maud land antarctica implications for dyke swarm emplacement abstract the mesozoic dyke swarms of western dronning maud land antarctica form a minor intrusive component of the karoo large igneous province five hundred and sixty one dykes were recorded intruding neoproterozoic gneisses and middle jurassic syenite plutons 40 ar 39 ar geochronology data reveal two temporally distinct components the 178 175 ma alkaline straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton and the 206 204 ma tholeiitic jutulrora dyke swarm found throughout the study area the straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction interpreted to be the result of high magma pressure equal to the maximum principal stress the jutulrora swarm displays a fan of dyke trends with dyke thickness and spacing increasing away from the inferred point of fan convergence anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the straumsvola area with the southern outer exposures of the jutulrora swarm exhibiting lateral magma transport although associated with a long lived local igneous centre comparison of palaeostress estimates for the straumsvola dyke swarm and contemporaneous dykes in ahlmannryggen and vestfjella indicates the presence of a regional scale radial stress system in western dronning maud land between 178 175 ma supporting a mantle plume origin for the karoo large igneous province

anisotropy of magnetic susceptibility in the ponta grossa dyke swarm brazil and its relationship with magma flow direction abstract measurements of anisotropy of magnetic susceptibility ams in 95 mafic dykes mainly tholeiites 10 200 m in width from the mesozoic ponta grossa swarm southern brazil revealed two main types of magnetic fabric type i fabric plane k 1 k 2 parallel to the dyke plane represents magma flow within the dykes whereas type ii plane k 1 k 3 parallel to the dyke plane is compatible with a fabric pattern reflecting vertical compaction of the magma column fabric type i dominates 51 of the dykes within the swarm whereas type ii 38 of the dykes concentrates mainly in the western region where the dykes intrude sediments considering the dykes with type i fabrics it is concluded that 58 of the dykes were fed by horizontal or sub horizontal k 1 inclinations less than 30 magmatic flow and 42 were fed by inclined to vertical k 1 inclinations more than 30 magma flow the latter are more frequent in the southeastern part of the swarm suggesting a magma source close to this area although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found

cyclic sedimentation in the shallow marine upper permian kennedy group carnarvon basin western australia abstract the upper permian kennedy group of the carnarvon basin western australia was deposited while the merlinleigh sub basin was undergoing thermal subsidence and the global climate was warming from the carboniferous permian glaciation to the mesozoic greenhouse conditions the kennedy group comprises siliciclastic sedimentary rocks dominated by sandstones in the lower part with coarsening up cycles from mudstones to very coarse sandstones and granule conglomerates in the upper part cycles were observed and logged in the field and although many types of cycles were found three main motifs describe most of the cycles cycle motifs are defined based on the dominance of the various facies mooka motif cycles are dominated by fine grained and bioturbated facies interpreted as deposition during relative sea level highstand with only thin laminated or cross bedded sandstones interpreted as deposited during relative sea level fall binthalya motif cycles are dominated by laminated and cross bedded sandstones and coolkilya motif cycles consist of alternating laminated and bioturbated beds a hierarchy of cycles was observed in the field and this correlated well with spectral analysis of logged parameters spectral analysis of section log data and data sets corrected for the effects of compaction and sedimentation rates detected regular cyclicity geochronology of the kennedy group is not well enough constrained to allow the cycle periods in time to be calculated but ratios of the different scale cycle thicknesses correlate well with ratios of the milankovitch orbital cycles that have been calculated for the permian the presence of regular cyclicity and even bed thicknesses across large distances are not consistent with tectonic or autocyclic models of cycle formation the cycles are more likely to have been caused by fluctuating eustatic sea levels perhaps enhanced by changing amounts or seasonality of precipitation both sea levels and climatic fluctuations would ultimately be controlled by the milankovitch orbital cycles

sea level curve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america at least 55 cycles of marine inundation and withdrawal are recognized in the mid desmoinesian to mid virgilian midcontinent outcrop sequence in north america they range from widespread major cycles classic cyclothems with deep water facies extending across the northern shelf through intermediate cycles persisting as marine horizons across the shelf to minor cycles developed on the lower shelf or as parts of major cycles biostratigraphic differentiation of the cycles should establish interbasinal correlation on a scale fine enough to allow evaluation of differential tectonics and sedimentation sequential groupings of cycles are more irregular than proposed megacyclothems or mesothems but they may be obscured by the distinctness of the major cyclothems estimates of cycle periods range from about 40 to 120 x 10 sup 3 yr for the minor cycles up to about 235 to 400 x 10 sup 3 yr for the major cyclothems the range for all cycles corresponds well to the range of periods of earth s orbital parameters that constitute the milankovitch insolation theory for the pleistocene ice ages and it further supports gondwanan glacial control for the pennsylvanian cycles even though the dominant period of the major pennsylvanian cyclothems is up to four times longer than the dominant 100 000 yr period inmore the pleistocene the shapes of both curves display rapid marine transgression rapid melting of ice caps and slow interrupted regression slow buildup of ice caps which suggest similar linkages between the climatic effects of the orbital parameters and ice cap formation and melting at the two different scales widely separated in time 28 references 3 figures less

slope instability in nicaragua triggered by hurricane mitch distribution of shallow mass movements the heavy rains associated with hurricane mitch triggered off a number of slope instability processes in several central american countries different instability processes have been acknowledged for the various mountainous regions of nicaragua an enormous movement of the casita volcano slopes resulted in numerous deaths and some deep movements have been reactivated on the other hand numerous shallow mass movements and debris flows have given rise to great material loss throughout a large part of nicaraguan mountains

depositional cycles composite sea level changes cycle stacking patterns and the hierarchy of stratigraphic forcing examples from alpine triassic platform carbonates carbonate platform deposits record a complex interplay of numerous geodynamic variables of which eustasy subsidence and sediment accumulation are prime factors in determining both the kilometer scale depositional sequence scale and meter scale depositional cycle scale stratigraphic packaging in this study we looked particularly at composite eustasy that is superimposed sea level fluctuations with different frequencies defined as orders and different amplitudes and the role it plays in the linkage between meter scale cyclic stratigraphy and kilometer scale sequence stratigraphy specifically we have investigated the relationship between low frequency third order 1 10 m y period depositional cycles and their component high frequency fourth 0 1 1 m y and fifth 0 01 0 1 m y order cycles through detailed stratigraphic analyses of alpine triassic platforms complemented by computer modeling on the basis of our results we suggest that in general there exists a hierarchy of stratigraphic forcing driven by composite eustasy that results in organized stacking patterns thickness subfacies character early diagenetic attributes of high frequency typically fourth and fifth order shallow water carbonate cycles dictated by low frequency third order relative sea level effects we suggest that systematic vertical changes in stacking patterns of high frequency cycles across a larger depositional sequence are due to systematic and predictable differences in depositional space available during the rising and falling stages of a relative third order sea level change we also suggest that these systematic variations in cycle stacking patterns will exist regardless of the mechanism responsible for generating the high frequency cycles be it an autocyclic or allocyclic mechanism this approach has major implications for the use of high frequency fourth and fifth order cycle characteristics to identify third order cycles in outcrops and cores of shallow water carbonates where stratigraphic control may be less than desirable this would constitute a valuable bridge between cyclic stratigraphy at the meter scale and sequence stratigraphy at the seismic scale we present two examples from triassic buildups of the alps the ladinian latemar buildup and the norian dolomia principale where a systematic succession of high frequency cycle stacking patterns and early diagenetic features exists within an overriding third order cycle sequence reflecting the interplay of short term high frequency fourth fifth order eustasy and long term low frequency third order eustasy in accordance with the hierarchy of stratigraphic forcing central to the interpretation of these examples is the demonstration that true eustatic rhythms are recorded in the high frequency cyclicity as verified by time series analyses and the use of fischer plots these examples can be modeled by computer under conditions of lag depth constrained sedimentation uniform subsidence and composite eustasy we also present two examples one from the alpine triassic the norian lofer cyclothems and one from the pleistocene of south florida that lack both a systematic succession of high frequency cycle stacking patterns and identifiable composite rhythms in the stratigraphic record despite the existence of composite high frequency eustasy in these examples we call upon 1 local tectonic forcing in the form of short term deviations in subsidence via faulting lofer example and 2 large differences in the relative amplitudes of different orders of high frequency sea level oscillations pleistocene example to explain the lack of composite rhythms and we present computer simulations to illustrate the concepts an understanding of composite relative sea level changes and the potential for a hierarchy of stratigraphic forcing provides the link between cyclostratigraphy and sequence stratigraphy and also has important implications regarding the stratigraphy of early diagenesis

petrology and isotope geochemistry of the mesoproterozoic anorthosite and related rocks of the kunene intrusive complex nw namibia abstract the intrusion of the large c 18 000 km2 anorthosite body of the kunene intrusive complex angola namibia which is associated with minor marginal granite bodies and syenite syenodiorite dykes marked the beginning of mesoproterozoic igneous activity in nw namibia anorthosites display an igneous mineral assemblage of dominating plagioclase together with minor olivine orthopyroxene or clinopyroxene fe ti oxides whereas biotite and pargasite are late magmatic phases apatite is a common accessory phase in regionally associated felsic rocks k feldspar plagioclase clinopyroxene and or hastingsite crystallized first followed by interstitial quartz hastingsite fe ti oxides and titanite the general geochemical characteristics for the anorthositic rocks of the kunene intrusive complex i e high al2o3 cao sr and eu intermediate to high mg numbers of 0 37 0 74 positive nd t values of 3 0 to 1 0 and initial sr ratios of 0 7028 0 7041 are in excellent agreement with their derivation from fractionated basaltic liquids the 18o values obtained for plagioclase 5 88 0 19 18o support their development from mantle derived magmas relic crustal xenoliths of calc silicate rocks and sr and nd isotope data however suggest that the anorthosite magma underwent minor crustal contamination which was most prominent during the early intrusion stage a u pb zircon age of 1376 2 ma was obtained for a felsic rock associated with the 1385 25 ma anorthosites which suggests that the two suites were emplaced during the same igneous event the geochemical and isotope signatures of the granites syenites and syenodiorites provide convincing evidence against consanguinity of the anorthositic and the felsic rock suites i e 1 a compositional gap exists between the major and trace element contents of the two suites 2 trace element and sr and nd isotopic data of the felsic rocks point to a crustal source nd 2 0 to 0 4 initial sr ratio 0 7024 0 7063 3 the ree patterns of the felsic rocks do not exhibit ubiquitous negative eu anomalies which would be expected from fractionation products of melts that previously formed extensive plagioclase cumulates and 4 18o values of magmatic feldspar from the felsic rock suite fall in a restricted range 7 20 7 92 that is about 1 6 higher than the average igneous plagioclase 18o of the anorthosites and suggests a formation of the felsic melts by anatexis of the lower crust

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

time partitioning in cratonic carbonate rocks dissimilarity in relations between thickness duration and accumulation rate in holocene sections metre scale phanerozoic cycles and epoch interval phanerozoic sequences reflects variation in the distribution of depositional and hiatal time at different scales of observation modern accumulation rates are strongly dependent on duration of deposition lower rates correspond to longer durations phanerozoic cycle thicknesses are similar to those of holocene sections suggesting that both formed while filling similar amounts of accommodation space however accumulation rates of phanerozoic cycles and sequences are about two orders of magnitude less than in modern settings and they show neither strong dependence on duration of accumulation nor significant variation over phanerozoic time hence net accumulation is primarily controlled by regional subsidence rate these relations also allow for determination of that fraction of cycle period recorded as rock and that corresponding to hiatal time at intercycle boundaries depending on subsidence rate and amount of compaction most cratonic carbonate units accumulated over 3 to 30 of the time represented by sequences in which they occur

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

cycle symmetry and its causes cisco group virgilian and wolfcampian texas abstract 181 transgressive regressive cycles composed of nonmarine and marine carbonate and siliciclastic rocks of the cisco group on the eastern shelf texas display complex characteristics at both hemicycle and full cycle scales they are delineated on the basis of successive changes of depositional environments stratal boundary relations and stratigraphic position transgressive and regressive stratigraphic environment gradients are defined as the magnitude of environmental shift divided by thickness for each hemicycle they indicate the rates of lateral environmental shifts during transgression and regression cycle symmetry index is defined as the ratio between transgressive and regressive stratigraphic environment gradients it provides a measure of stratigraphic response to controlli g processes most cisco cycles are asymmetrical with steep transgressive and shallow regressive stratigraphic environment gradients indicating cyclic but asymmetrical variations of major allogenic processes over a cycle interval these systematic variations are delineated from climate sensitive rock types stratal thickness and sedimentary features the relative dominance of processes controlling accommodation i e glacio eustasy and basement subsidence versus those controlling sedimentation i e climate upland sediment yield and sediment supply at the depositional site determines cycle symmetry a formulated relation between cycle symmetry index i e stratigraphic response and rates of these processes demonstrates that transgressive sedimentation is controlled by sediment supply whe eas regressive sedimentation is controlled by accommodation this relation is potentially useful in quantitatively predicting stratigraphic responses to specific allogenic processes in cyclic sedimentation five cisco cycle types defined by the type of component lithofacies display the stratigraphic response mainly to noncyclic allogenic and autogenic processes the cycle types have varying magnitude thickness and symmetry they also vary in lateral extent and in abundance a process response model of cyclic sedimentation of the cisco group on the eastern shelf is established it emphasizes the interplay between autogenic and allogenic processes at the sub cycle scale understanding interactions among glacio eustasy climate shelf subsidence sediment supply and depositional dynamics during various stages of transgression and regression is central to a clearer comprehension of the observed variations in cycle characteristics

the genetic relationship between mafic dike swarms and plutonic reservoirs in the mesozoic of central chile 30 33 45 s insights from ams and geochemistry five mafic dike swarms between 30 and 33 45 s were studied for their geochemical signature and kinematics of magma flow directions by means of ams data in the coastal range of central chile 33 33 45 s middle jurassic dike swarms concon and cartagena dike swarms cmds and crmds respectively and an early cretaceous dike swarm el tabo dike swarm etds display the presence of dikes of geochemically enriched high ti and depleted low ti basaltic composition these dikes show geochemical patterns that are different from the composition of mafic enclaves of the middle jurassic papudo quintero complex and this suggests that the dikes were injected from reservoirs not related to the plutonic complex the mantle source appears to be a depleted mantle for jurassic dikes and a heterogeneous enriched lithospheric mantle for cretaceous dikes in the etds vertical and gently plunging magma flow vectors were estimated for enriched and depleted dikes respectively which suggest together with variations in dike thickness that reservoirs were located at different depths for each dike family in the elqui dike swarm eds and limari mafic dike swarm lmds geochemical patterns are similar to those of the mafic enclaves of the middle jurassic plutons in the lmds east to west magma flow vectors are coherent with injection from neighbouring pluton located to the east in the eds some dikes show geochemical and magma flow patterns supporting the same hypothesis accordingly dikes do not necessarily come from deep reservoir they may propagate in the upper crust from coeval shallow pluton chamber

stability of the astronomical frequencies over the earth s history for paleoclimate studies the expected changes over the past 500 million years in the principal astronomical frequencies influencing the earth s climate may be strong enough to be detectable in the geological records and such effects have been inferred in several cases calculations suggest that the shortening of the earth moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession from 54 to 35 41 to 29 23 to 19 and 19 to 16 thousand years over the last half billion years at the same time the precessional constant increased from 50 to 61 arc seconds per year the changes in the frequencies of the planetary system due to its chaotic motion are much smaller their influence on the changes of the periods of climatic precession obliquity and eccentricity of the earth s orbit around the sun can be neglected eccentricity periods used for quaternary climate studies may therefore be considered to have been more or less constant for pre quaternary times

geochemistry isotope systematics and petrogenesis of the volcanic rocks in the zhongtiao mountain an alternative interpretation for the evolution of the southern margin of the north china craton abstract the xiong er volcanic rocks constitute a large paleo mesoproterozoic volcanic belt along the southern margin of the north china craton which are composed primarily of basaltic andesites and andesites with minor dacites and dacitic rhyolites the basaltic andesites and andesites from the xiong er volcanic rocks in the zhongtiao mountain show consistent nd t la nb and th nb values irrespectively of sio 2 which preclude significant crustal contamination during ascent both of these volcanic rocks share similar incompatible ratios e g la sm zr nb la nb indicating that the basaltic andesites and andesites originated from the same source the basaltic andesites are characterized by high mg number 45 55 precluding the possibility of their derivation from the melting of the lower crust variable ti eu zr sm and nb la ratios for the basaltic andesites with higher partial melting degree suggest that amphiboles remained as residues in the magma source indicating that the xiong er volcanic rocks were derived from hydrous magma non radiogenic nd isotopes for the xiong er volcanic rocks are inherited from a mantle source which had been enriched by the subduction related crustal recycling during archean to paleoproterozoic time basaltic andesites show hfse enrichments especially nb 4 7 ppm and high fe ti contents comparable with nb enriched basalts suggesting that the xiong er volcanic rocks were derived from a metasomatized mantle source similar to that of nb enriched basalts therefore the mantle source of the xiong er volcanic rocks in the zhongtiao mountain has been enriched in fe ti oxides and hfse by equilibrium reactions with the slab melts during late archean and paleoproterozoic subduction processes as evidenced by nd isotopes in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative anomalies on the nb ta ti similar to arc related volcanic rocks produced by the hydrous melting of the metasomatized mantle wedge in the paleo mesoproterozoic the xiong er volcanic rocks in the zhongtiao mountain were produced by the slab dehydration induced melting of an existing metasomatized mantle source and the fluids from the slab dehydration introduced significant lile and lree to the source masking its inherent hfse enriched characteristics the arc related characteristics of the xiong er volcanic rocks in the zhongtiao mountain suggest the xiong er volcanic belt constructs a subduction zone along the southern margin of the north china craton in the paleo mesoproterozoic implying that the craton may have recorded the outbuilding history of the columbia supercontinent during paleo mesoproterozoic time

glacial eustatic control of virgilian cyclothems in north central texas virgilian strata in north central texas consist of repetitive sedimentary sequences called cyclothems these cyclothems were caused by waxing and waning of glaciers in the southern hemisphere gondwana during the transgressive phase deltas were drowned forming estuaries and the oxygen minimum zone responsible for the black shales in the midland basin rose to a position well up on the shelf area resulting in a sequence of lithofacies and corresponding biofacies which indicate a water column with stratified dissolved oxygen content during early regression the oxygen minimum zone and the characteristic lithofacies and biofacies began retreating toward the midland basin deltas began prograding well out onto the outer shelf and the resulting lithofacies and biofacies depended upon the relationship between paleogeography and the prograding deltas during maximum regression the oxygen minimum zone was restricted to the midland basin and the shelf edge carbonates were subaerially exposed resulting in extensive freshwater diagenesis which was responsible for enhancing secondary porosity end of article last page 142

glacial eustatic sea level curve for early late pennsylvanian sequence in north central texas and biostratigraphic correlation with curve for midcontinent north america at least 30 transgressive regressive cycles of deposition are recognized from the upper desmoinesian east mountain shale to the mid virgilian wayland shale in north central texas maximum regressive deposits are typically paleosol mudstones and fluvial sand stones maximum transgressive deposits are typically widespread ammonoid bearing conodont rich dark phosphatic shales in more major cycles and persistent fossiliferous shales or limestones overlying terrestrial deposits in more minor cycles delta complexes dominate the regressive sequences of many cycles using biostratigraphic criteria of first last sole or acme occurrence of ammonoid conodont and fusulinid taxa we correlate 17 cycles in the texas sequence directly with 17 glacial eustatic cycles of similar magnitude in the northern midcontinent this correlation suggests that glacial eustacy was the basic control over the cyclic sequence in texas that tectonic masking of the eustatic signal by nearby orogenic movement in texas was negligible and that delta shifting though conspicuous was only a secondary control over the cyclicity there minor cycles recognized between the correlated cycles also match well enough between texas and the midcontinent to further discount potential tectonic or deltaic masking of glacial eustatic cyclicity this strengthens the likelihood of correlating glacial eustatic events across larger parts of north america and perhaps with other parts of the world

documenting the reliability and utility of the γ method as applied to cyclic sections using forward modeling abstract the problem of determining if cyclic sections which are older than 5 m y in age were deposited in response to orbital control is discussed gamma analysis a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration is presented as a possible solution to this problem while the assumptions of analysis are generally not met by geologic data the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control we address the problem of assessing the reliability of results by performing analyses on three cyclic time series generated by a forward model our results show that exact accumulation rates are not likely to be obtained by the method however the accumulation rates obtained markedly improve the time scale resulting in a spectrum from which one could distinguish orbital forcing if present only when spectra of cyclic data show predicted orbital peaks can analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

a record of glacial aridity and milankovitch scale fluctuations in atmospheric dust from the pennsylvanian tropics abstract atmospheric dust is an important climatic agent as well as a valuable climatic archive this study documents systematic variations in a single upper pennsylvanian shallow water carbonate buildup of the midland basin west texas that was isolated from fluvio deltaic input and consists almost entirely of carbonate bounded by sequence boundaries thin 0 02 to 1 m intervals of dark pyritic siliciclastic mudrock with subtle pedogenic features occur commonly at the sequence boundaries and are interpreted as loessite deposited during lowstands primarily on the basis of paleogeographic isolation of the buildup the silicate mineral fraction smf extracted from the underlying carbonate which includes quartz clay and trace feldspars and heavy minerals varies in concentration weight from 0 015 to 6 69 in the carbonate this smf contains both fine grained detrital and authigenic silica interpreted to be eolian on the basis of geochemical petrographic and grain size characteristics as well as paleogeography hence the smf is considered a proxy for atmospheric dust whole rock geochemistry suggests that the dust was at least partly derived from uplifts of the ancestral rocky mountains with a possible minor contribution from active volcanic centers located to the present day south and southwest the estimated mean accumulation rate mar of dust in highstand carbonate facies full interglacial relative to carbonate facies that formed during relative fall in sea level suggest that dust flux increased 4 to 10 times accompanying the transition to glacial conditions mars in the siliciclastic mudrock which accumulated at lowstand full glacial to incipient transgression suggest glacial phase increases in dust flux of nearly 400 to 4000 times relative to full interglacial conditions spectral analyses of dust abundance through the study section show variations consistent with milankovitch forcing

accommodation and sedimentary architecture of isolated icehouse carbonate platforms insights from forward modeling with carb3d abstract a fundamental control on the evolution of carbonate platforms is accommodation the space available for sediment deposition here we adopt a sensitivity analysis approach using the process based forward model carb3d to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy subsidence and surface dissolution sediment production the mechanism invoked to fill accommodation is also investigated fourth order milankovich driven sea level cycles dominate icehouse carbonate sedimentology with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present both overwhelm higher frequency lower amplitude fifth order cycles which are rarely preserved on the platform top subsidence is a fundamental control on sedimentary stacking patterns sequence boundary frequency sbf and platform geometry with rapid subsidence increasing the preservation of platform top sequences most icehouse platforms are subject to extended periods of platform top exposure missed beats when the platform top remains emergent at a sea level maximum are common and create non milankovich sbfs furthermore during platform emergence surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event with surface dissolution rates in humid climates comparable to moderate rates of subsidence however accommodation generation by surface dissolution is self limiting in as much as accommodation generated by dissolution is rapidly refilled on flooding sediment production is a key control on unfilled accommodation which our simulations suggest is considerable on icehouse platforms increased shallow water 3 m depth sediment production enhances intra platform bathymetric relief between the margin and interior this is driven in large part by a higher sedimentation rate at the margin than in the interior platforms can be flooded to depths up to 45 m during marine transgressions and accommodation often remains unfilled there are two distinct phases of platform top sedimentation during the transgression and early highstand platform margins accumulate at a greater rate than interior sediments generating differential intra platform bathymetry subsequently accommodation in the interior fills by forced regression and progradation from the margins into the platform interior numerical forward models can augment our understanding of icehouse carbonate platform evolution by identifying the nature of the sedimentary response to individual controls they can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms such as the large hydrocarbon fields in the north caspian and s e asia

anisotropy of magnetic susceptibility of hannuoba basalt northern china constraints on the vent position of the lava sequences 1 anisotropy of magnetic susceptibility ams was determined for hannuoba basaltic samples from 17 flows 21 4 0 7ma distributed in three sections jianshaba yuershan and manjing around zhangbei northern china the results show normal flow fabrics with horizontal foliation planes the typical ratios of the principal ams axes are 1 01 for the magnetic lineation l and 1 02 for the foliation f respectively magnetic hysteresis loop isothermal remanence thermomagnetic properties and optical observations indicate that pseudo single domain psd low titanium magnetite particles dominate the low field susceptibility of samples thus the ams of the magma at this area is believed to be originated from the lineation of the low titanium magnetite particles driven by the lava flow since the kmax trends coincide well with the observed flow directions the intersection of the mean kmax trends from these three sites perfectly defines the previously known vent position of the lava sequences

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

stratigraphic and sedimentary records of the rift to drift evolution of the northern north china craton at the paleo to mesoproterozoic transition article i nfo this paper presents a stratigraphic and sedimentary study of late paleoproterozoic to early mesoproterozoic successions of the northern margin of the north china craton including the changcheng and nankou groups a reconstruction of the tectono sedimentary history of these two groups demonstrates that the northern north china craton experienced a two stage evolution as recorded by the changcheng group early synrift subsidence and sedimentation were controlled by normal faults and accompanied by volcanism in contrast a late postrift stage was characterized by broad subsidence and widespread occurrence of shallow marine clastic and carbonate rocks a transgressive unconformity is identified and together with its time equivalent conformable contact clearly separates the synrift and postrift sequences the diachronous unconformity can be related to continental rifting and the separation of the north china craton from the adjacent india craton at roughly 1600 ma and is thus interpreted as a breakup unconformity this age of ca 1600 ma is constrained by newly reported precise shrimp u pb zircon ages of transgressive deposits in the overlying mesoproterozoic dahongyu and gaoyuzhuang formations just coincident with the paleoproterozoic to mesoproterozoic transition the 1600 ma time estimate for the tectonic departure of the north china craton from the india craton is comparable with the timing for the breakup of columbia a pre rodinia supercontinent containing almost all of the earth s continental blocks

are coal balls rare a cyclostratigraphic analysis of coal ball occurrence in north america abstract from the perspective of phanerozoic time coal balls are rare apparently limited to a 24 m y interval 323 299 ma in the pennsylvanian and earliest permian yet within this interval coal balls occur in many coals approximately 82 transgressive regressive sedimentary cycles have been described for the midcontinent illinois and appalachian basins of north america during the mid to late pennsylvanian one third 27 82 have coal balls including 57 of major cycles 36 of intermediate cycles and 16 of minor cycles coal ball occurrence in the donets basin is similar over an interval of about 4 m y 315 311 ma latest bashkirian to mid moscovian 39 11 28 of transgressive regressive cycles have coal balls as north american paleoclimate became drier tree ferns replaced lycopsids as the dominant plant in peat swamps at the desmoinesian missourian boundary and coal ball occurrence declined overall 47 of cycles 19 40 with lycopsid or cordaitean dominance have coal balls whereas 19 8 42 of cycles with tree fern dominance have coal balls p this pattern may reflect the relative abundance of coal in the late pennsylvanian with drier paleotropical climates in the missourian virgilian leading to less paleotropical coal fewer mines and a lower probability of discovering missourian and virgilian coal balls however average cycle duration decreased from 150 kyr in the desmoinesian to 100 kyr in the missourian and virgilian and faster rates of relative sea level rise could have affected coal ball abundance particularly if coal balls formed in marine swamps with more rapid transgression leading to thinner coals or to freshwater coals directly overlain by marine sediments rygel et al 2008 suggested that missourian virgilian cycles recorded more erosional relief than desmoinesian cycles consistent with increased amplitude and rates of sea level rise during each glacial eustatic cycle changes in missourian virgilian cycles may have been augmented by stratigraphic attenuation as sediment filled the antecedent topography

crystallization processes in an artificial magma variations in crystal shape growth rate and composition with melt cooling history a large 4 8 m3 1 3x107 g artificial mafic melt with a bulk composition similar to that of a basalt but with a high cao content of 17 wt was generated during a demonstration of in situ vitrification and was allowed to cool naturally during the melting process convection was vigorous resulting in a chemically and thermally homogeneous melt body once heating was complete the cooling rate was rapid with the temperature dropping from 1500 c to 500 c in 6 days within the interior of the 3 m diameter 1 5 m thick body a 20 h period of constant temperature 1140 c observed during colling was the result of latent heat released by widespread crystallization the final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar with a subordinate amount of potassic feldspar plus a small amount of evolved glass the compositions and proportions of phases agree well with those predicted by the melts thermodynamic model thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals textural investigation of core samples reveals large up to 1 cm in length acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals 500 m in length crystal shape and pyroxene composition vary as a function of position within the solidified body as a function of cooling rate both crystal size and degree of crystallinity are highest in the central most slowly cooled parts of the rock crystal shape is characterized by tabular equilibrium growth forms in the slowly cooled areas grading to highly skeletal dendritic forms at the rapidly cooled edges of the body the pyroxene crystals are dominantly homogeneous diopside but crystals are characterized by thin fe rich hedenbergitic rims these rims were deposited when mg in the melt was depleted by diopside growth and melt temperature had cooled sufficiently to allow fe rich pyroxene growth crystal growth rates can be calculated based on thermal behavior of the melt reinforced by thermodynamic modelling and are determined to be between 10 7 and 10 8 cm s in the central part of the melt these estimates agree well with growth rates in natural systems with similar cooling rates pyroxene crystals that formed at a higher cooling rates are characterized by higher al and lower mg contents relative to tabular equilibrium crystalline forms presumably as a result of disequilibrium melt compositions at the crystal melt interface

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

geochemical ar 40 ar 39 geochronological and sr nd isotopic constraints on the origin of paleoproterozoic mafic dikes from the southern taihang mountains and implications for the ca 1800 ma event of the north china craton abstract the wide occurrence of paleoproterozoic mafic dikes in the basement of the trans north china orogen provides important constraints on the major 1800 ma tectonothermal event of the north china craton these mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites and can be geochemically classified into three groups the groups 1 and 2 rocks are characterized by high total feo contents 12 69 15 61 of which the group 1 rocks are more enriched in lile and lree contents and depleted in hfse th nb n 2 5 3 8 la yb cn 3 0 5 8 nb la n 0 27 0 38 and have more radiogenic nd isotopic ratios nd t 3 40 to 5 14 than the group 2 rocks th nb n 0 8 1 2 la yb cn 1 5 2 1 nb la n 0 65 0 87 nd t 0 60 to 1 67 in contrast the group 3 rocks are typified by low feot 7 86 11 04 and high mgo 5 62 9 56 contents higher la yb cn 4 22 7 13 ratios less radiogenic nd isotopic ratios nd t 2 75 to 5 52 and more significant th u and nb ta depletion are also apparent features 40ar 39ar geochronology of three representative samples from each group yielded plateau ages of 1780 7 0 5 ma 1765 3 1 1 ma and 1774 7 0 7 ma respectively these elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement instead they most likely originated from variable sources under different degrees of partial melting the group 1 rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction related fluids whereas the group 2 rocks originated from a hybridized source involving ca 40 subduction modified lithospheric mantle similar to the group 1 and ca 60 n morb component in contrast the group 3 rocks show significant th u depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction based on all the available data we propose a tectonic model of a full subduction syncollision postcollision rifting cycle for the evolution of the ncc between 1870 and 1765 ma a variety of source regions beneath the southern taihang mountains initially developed during the period of subduction collision and subsequent extension rifting resulted in the melting of the subduction modified lithospheric mantle which produced widespread paleoproterozoic mafic dikes on the trans north china orogen in response to upwelling of convective mantle

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

whither cyclostratigraphy testing the gamma method on upper pleistocene deep sea sediments north atlantic deep sea drilling project site 609 1 abstract gamma analysis a procedure for tuning cyclic sediments in which gamma is defined as 1 sedimentation rate was applied to the upper pleistocene gray scale record of deep sea drilling project site 609 from the north atlantic the spectrum of the gamma tuned time series produces eccentricity obliquity and precession index signals when the primary periodicity is tuned to the precessional beat frequency however the timescales are significantly too long suggesting that some of the primary cycles used in the gamma tuning procedure have higher frequency than the precessional beat by filtering the specmap calibrated time series at i 0 12 kyr the spurious cy cles are removed from the tuning procedure in this case the resulting gamma corrected spectra look much more like that of the specmap tuned time series however the time series remains slightly too long forward modeling is consistent with the results from modeling the core that is the gamma method is quite robust at revealing orbital periodicity in an orbitally controlled re cord for which sedimentation rates are facies dependent however the method does not produce an accurate time series criteria for complete rejection of gamma results are clarified as either 1 least squares results include negative gamma values or 2 the resulting spectrum does not show an or bital signature variation on sedimentation rates greater than 5 result in incorrect time series however variations on sedimentation rates as high as 15 generally reproduce strong orbital spectra and occasionally the method can also generate an orbital spectrum with up to 25 noise the method is not as robust in predicting gamma values or sedimentation rates although it generally distinguishes facies with relatively high sedimentation rates from facies with relatively low sedimentation rates the values predicted may be incorrect by a factor of2 thus while gamma tuning of the time series is an improvement over observed thicknesses the resulting timescale is not correct the implications of these results for cyclostratigraphy are significant the generation of an orbital spectrum from a time series that is demonstrably incorrect casts doubt on cyclicity as a dating tool

spectral analysis of the middle triassic latemar limestone abstract the middle triassic latemar limestone in the dolomites region northern italy consists of hundreds of vertically stacked meter scale platform carbonate cycles each cycle is composed of a shallow marine subtidal carbonate unit with a thin centimeter scale vadose diagenetic dolomite cap these latemar couplets are the products of repeated episodes of submergence emergence with durations estimated at 20 000 yr the couplets are arranged in upward thinning bundles of five indicating the presence of a 100 000 year modulating component we examined this high frequency cycling in the platform buildup using a variety of classical and advanced spectral analysis techniques we found si nificant evidence for the presence of a milankovitch forcing signal in this cycling we also present evidence that the adjacent coeval basinal facies the livinallongo formation exhibits a similar bundling pattern in its cm scale hemipelagic carbonate shale alternations we conclude that milankovitch forced small amplitude sea level oscillations were the most likely cause of these middle triassic carbonate rhythms

the expected duration of upward shallowing peritidal carbonate cycles and their terminal hiatuses meter scale accommodation cycles which dominate the stratigraphic record of peritidal carbonate accumulation might provide a time scale of unusually high resolution but the duration of the hiatuses that cap the resulting cyclothems is difficult to determine thousands of rates of peritidal accumulation from many different carbonate sections have been combined according to the length of the time span of measurement to describe a hypothetical average section because all sections are interrupted by hiatuses the average accumulation rate falls progressively with increasing time span and most steeply at the time spans that capture the highest proportion of hiatuses the process of combining rates from many sections cancels local differences in the distribution of hiatuses only hiatuses that have similar spacing and duration in a majority of sections will stand out after compilation the size and position of a pronounced inflection in the combined plot of accumulation rates against time span identifies a dominant cycle period of 100 000 yr of which the terminal hiatus accounts for 80 90 the expected cyclothem is 10 m thick because peritidal accommodation cycles begin and end with the sediment surface at sea level the expected cycle period can be estimated by finding the time span at which accommodation and accumulation rates balance the maximum duration of the hiatus is the time span at which rates of subsidence and sea level fall balance compilations of many empirical measurements were again employed to determine the relationship between time span and the rates of subsidence sea level change and net accommodation this second method confirms the results of the first and can be modified to predict the expected cyclothem in different climatic tectonic and depositional settings because quaternary data dominate short term rates the hypothetical average section best describes a passive margin in an icehouse climate with strong glacio eustatic fluctuations in sea level after modification for an extreme greenhouse climate devoid of ice caps the second method predicts that 20 000 yr accommodation cycles will be preferentially recorded the expected cyclothem is 2 m thick and accounts for about 50 of the cycle period increased subsidence rates whether on active margins in the initial phase of passive margin formation or at the seaward edge of a platform tend to extend the limit of cycle duration increase cyclothem thickness and partition less time to the bounding hiatus increased subsidence however raises the risk that the sediment surface will drown

thick dyke emplacement and internal flow a structural and magnetic fabric study of the deep seated dolerite dyke of foum zguid southern morocco 1 knowledge on forced magma injection and magma flow in dykes is crucial for the understanding of how magmas migrate through the crust to the earth s surface because many questions still persist we used the long thick and deep seated foum zguid dyke morocco to investigate dyke emplacement and internal flow by means of magnetic methods structural analysis petrography and scanning electron microscopy we also investigated how the host rocks accommodated the intrusion regarding internal flow 1 important variations of the rock magnetic properties and magnetic fabric occur with distance from dyke wall 2 anisotropy of anhysteretic remanent magnetization reveals that anisotropy of magnetic susceptibility ams results mainly from the superposition of subfabrics with distinct coercivities and that the imbrication between magnetic foliation and dyke plane is more reliable to deduce flow than the orientation of the ams maximum principal axis and 3 a dominant upward flow near the margins can be inferred the magnetic fabric closest to the dyke wall likely records magma flow best due to fast cooling whereas in the core the magnetic properties have been affected by high temperature exsolution and metasomatic effects due to slow cooling regarding dyke emplacement this study shows that the thick forceful intrusion induced deformation by homogeneous flattening and or folding of the host sedimentary strata dewatering related to heat as recorded by thick quartz veins bordering the dyke in some localities may have also helped accommodating dyke intrusion the spatial arrangement of quartz veins and their geometrical relationship with the dyke indicate a preintrusive to synintrusive sinistral component of strike slip

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

u pb id tims baddeleyite ages and paleomagnetism of 1 79 and 1 59 ga tholeiitic dyke swarms and position of the rio de la plata craton within the columbia supercontinent abstract the tandilia terrane southernmost fringe of the rio de la plata craton is an igneous and metamorphic complex produced by an accretionary orogeny 2 25 2 02 ga calc alkaline acidic dykes with e w strike and a major shear zone with similar orientation are related with the late orogeny stage as supported by field relations in a previous study the acid dykes gave 40 ar 39 ar ages of 2007 24 ma to 2020 24 ma a n and nw trending tholeiitic dyke swarm tandil swarm is also present in the tandilia terrane one sample from the nw trending subset previously gave a u pb id tims baddeleyite age of 1588 11 ma new precise u pb id tims baddeleyite dating of both n and nw trending tandil dykes yielded crystallization ages of 1589 3 ma 1588 3 ma and 1588 3 ma significantly older tholeiitic dykes known as the florida swarm occur in the northern rio de la plata craton for which a u pb id tims baddeleyite age of 1790 5 ma was previously reported consequently intermittent rifting 1 79 1 59 ga took place after tectonic stabilization of the late paleoproterozoic lithosphere proto rio de la plata craton the available geochemical data for the 1 59 ga tandil dykes define low and high tio 2 trends although only the low tio 2 subgroup is firmly dated both the tandil and florida dykes have geochemical and nd sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects the tandil dykes may be linked with the 1 57 0 02 ga capivarita anorthosite which occurs to the east of the northern part of the craton correlatives on other crustal blocks may include those in baltica such as bimodal rock association including the breven hallefors and aland aboland diabase dykes and in the reconstructed gawler craton nw laurentia dolerites bimodal magmatism and iocg deposits contemporary within plate bimodal associations are also present in the sw amazonian craton paleomagnetic data for the 1790 ma florida dykes allows three possible reconstruction scenarios for the position of rio de la plata craton i in the southern hemisphere linked to the pampia terrane and the amazonia rio apa landmass as close as they are in present day ii in the northern hemisphere as a nearest neighbor to the reconstructed nw laurentia and gawler blocks proto australia or iii in the northern hemisphere near the boundary of amazonia proto amazonian craton and baltica positions 2 and 3 are consistent with all three criteria paleomagnetic poles and proximity to matching ca 1790 ma and 1590 ma mafic magmatism our data are consistent with the idea that the columbia supercontinent started major break up at 1 59 ga

transgressive wave ravinement on an epicontinental shelf as recorded by an upper pennsylvanian soil nodule conglomerate sandstone unit kansas and oklahoma u s a abstract a thin 3 25 cm persistent conglomerate sandstone unit occurs at the base of the transgressive systems tract of an upper pennsylvanian cyclothem below a transgressive limestone in outcrop and subsurface in se kansas and ne oklahoma it has an erosional base and a sharp top and is composed of calcitic clasts some shell fragments and quartz sand and rare coal fragments clasts are rounded equant to elongate coarse sand to pebble sized and moderately to well sorted they are micritic to microsparitic or radial fibrous calcite grains and pisoids micritic and microsparitic clasts are heterogeneous and contain rounded to elongate moulds and radiating or concentric spar or micrite filled cracks pisoids have micrite cores and superficial calcite or clay rims the clasts have similar texture and composition to the calcitic nodules and rhizoconcretions in underlying paleosols and thus were probably derived from these soil nodules upper shoreface erosion during shoreline transgression excavated the underlying paleosols and eroded coeval transgressive deposits landwards of the shoreline the eroded sediments were then reworked and transported to the lower shoreface and inner shelf by storm return flows to be deposited as a soil nodule conglomerate sandstone unit its base is an amalgamated surface of wave ravinement and initial transgression as well as the lower sequence boundary the widespread conglomerate sandstone unit suggests extensive transgressive wave ravinement on an epicontinental shelf the transgressive record composed of upward deepening dominantly fine grained lithofacies defines a new type t c4 of transgressive record which is composed of mixed carbonate and siliciclastic lithofacies with a simple transgressive lag on a low relief shelf

dynamics of cyclic carbonate deposition and biotic recovery on platforms during the famennian of late devonian in guangxi south china constraints from high resolution cycle and sequence stratigraphy to better understand the dynamics of platform carbonate deposition evolution and biotic recovery during the famennian high resolution cycle and sequence stratigraphic analyses were carried out upon three platform sections one in ramp and two in rimmed platforms along with a biostratigraphically well constrained basinal section in guangxi south china four main facies and fourteen lithofacies are identified in the three isolated platforms these are vertically arranged into decimetre to metre scale shallowing upward peritidal and subtidal cycles or parasequces in which the peritidal cycles predominate over the lower middle famennian six third order sequences s1 to s6 are identified based on the cycle stacking patterns as illustrated by fischer plots and vertical facies changes these sequences are temporally calibrated through correlation with basinal sequences constrained with conodont zonation and match well the six main famennian transgressive regressive cycles t1 to t6 in euramerica and elsewhere indicating a eustatic control however an opposite longer term trend in accommodation change s3 to s4 between platform interior and deeper open shelf to basin settings during the middle famennian suggests a differential subsidence across the platform basin section likely driven by block tilting thus both eustatic fluctuations and local tectonism had played an important role in platform development and evolution after the frasnian famennian biotic crisis main marine benthos preferentially recovered in the normal marine niches on the nearshore open shelves and radiated gradually into offshore restricted platforms brachiopods notably rhychonellides were the taxa recovering the earliest s1 in the normal or near normal marine conditions then echinoderms s3 stromatoporoids upper s3 and corals s5 stepped in episodically however these benthos generally appeared later in the restricted platform interiors only as if the ecological conditions were improved well enough for their colonization therefore the biotic recovery processes were diachronous both taxonomically and ecologically specified

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

estimation of duration of subaerial exposure in shallow marine limestones an isotopic approach abstract subaerial exposure surfaces in shallow marine limestones may represent a significant period of nondeposition and or erosion the minimum duration of subaerial exposure is estimated through modeling the maximum whole rock 13c depletion toward exposure surfaces in vadose meteorically altered limestones the model simulates the water rock interaction that caused the depletion and calculates the volume of meteoric water needed to achieve the observed 13c value then the duration of vadose meteoric diagenesis which approximates exposure duration is the volume of water divided by the infiltration rate of vadose water sensitivity tests indicate that estimated duration is moderately sensitive to many input parameters particularly the isotope composition and concentration of total dissolved carbon in vadose water sediment porosity and infiltration rate of vadose water the method is mostly effective in estimating exposure durations of less than 400 ky the estimated durations of nine subaerial exposures in pleistocene reef tracts of barbados west indies agree well with available radiometric dates application of the method to subaerial exposures in mississippian pennsylvanian and cretaceous shallow marine limestones confines their durations to 21 54 ky and distinguishes exposures of different durations the proposed method offers a potential tool and direction in estimating minimum duration of stratal hiatuses in meter scale stratigraphic studies

milankovitch climatic signals in lower triassic olenekian peritidal carbonate successions nanpanjiang basin south china abstract meter scale peritidal carbonate strata from an isolated platform in the nanpanjiang basin of south china were tested for the presence of periodic or quasi periodic climatic signals these signals provide information on factors controlling cyclic sedimentation in the equatorial eastern tethys during the early triassic greenhouse period parasequences are composed of shallowing upward successions of subtidal facies including skeletal packstone and grainstone calcimicrobial boundstone and oolitic packstone and grainstone intertidal laminated lutite and ribbon rock and rare supratidal microbial laminite parasequence stacking patterns display three orders of cyclicity suggesting hierarchical stratigraphic relationships gamma analysis of kominz and bond earth planet sci lett 48 1990 233 244 was used to estimate facies dependent thickness time conversion factors i e s and to construct tuned and untuned time series for two stratigraphic sections spectra of these time series indicate the presence of quasi periodic signals with prominent short eccentricity 94 9 131 2 kyr short obliquity 35 8 kyr and long precessional index 21 2 kyr peaks and minor long eccentricity 412 9 kyr long obliquity 45 3 kyr short precessional index 17 7 kyr and constructional tone 9 7 kyr peaks when calibrated on the tuned spectra thus milankovitch climatic forcing probably greatly influenced sedimentation the calibration indicates that the subtidal facies has a sedimentation rate of 24 6 30 7 cm kyr and the intertidal supratidal facies has a rate of 2 7 6 0 cm kyr the estimated duration of deposition of the two sections is 1139 1423 kyr and corresponds to a stratigraphic completeness of 32 92 the completeness is much greater than that of icehouse stratigraphic records we speculate that variations in carbonate productivity and environmental conditions driven by milankovitch climatic forcing combined with low amplitude sea level fluctuations were likely major controls on cyclic sedimentation furthermore evolutional spectra of the two sections show that dominant milankovitch climatic forcings varied from short eccentricity obliquity to long precessional index during the course of sedimentation some spectral differences between the two sections suggest variations in depositional recording mechanisms of milankovitch climatic signals between platform interior and windward platform margin

controls on sedimentation and cyclicity of the boquillas and equivalent eagle ford formation from detailed outcrop studies of western and central texas u s a

a new method of testing periodicity in cyclic sediments application to the newark supergroup abstract we present a new quantitative method for analyzing cyclic sediments the gamma method gamma time thickness tests the assumption that the cycles are periodic and provides quantitative estimates for relative accumulation rates of the different sedimentary facies that compose the cycles as such it provides a means of estimating relative time at very high resolution application of the gamma method to hettangian sediments of the newark supergroup indicates that the primary cycles were deposited in response to precessional climatic forcing and that these cycles were modulated by an eccentricity cycle with period ratios very similar to those of today results of gamma analysis also suggest that relative accumulation rates of the laminated deep water lacustrine facies was about six times slower than that of the shallow water facies

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

quantitative analysis of a cyclic peritidal carbonate sequence the middle and upper devonian lost burro formation death valley california a possible record of milankovitch climatic cycles abstract the middle and upper part of the type section of the lost burro formation is composed of 186 m of peritidal limestone dolomite and quartz arenite meter scale analysis of lithology sedimentary structures fossil assemblages and microfacies delineated 54 primary depositional cycles of subtidal intertidal and supratidal facies and a larger scale trend of apparent sea level change gamma analysis of kominz and bond 1990 tests the assumptions that cycles have equal durations and that each facies has a unique time thickness relation in the primary gamma and spectral analysis signals of milankovitch climatic cycles are indicated on the spectrum of the spatial facies curve but gamma tuning failed to improve the spectrum in subsequent analysis the 54 cycles are subdivided into fi e subgroups of different mean durations based on cycle period distributions gammas of three subgroups are strikingly constant indicating that they least violate the gamma assumptions the grouping of cycles suggests that the primary cycles do not have the same duration gammas are used to convert the measured section into two time series one is gamma uncorrected uniformly stretched another gamma corrected nonuniformly stretched after calibrating the gamma time unit as 18 4 ky eight statistically significant spectral peaks on the gamma corrected spectrum match the predicted 377 ma milankovitch eccentricity 2035 413 131 and 95 ky obliquity 32 ky and precession index 20 17 and 9 ky the constructive tone peaks the match suggests that milankovitch climatic forcing wa an important control on the lost burro carbonate deposition the effective accumulation rates of the subtidal 280 mm ky intertidal 160 190 mm ky and supratidal 60 70 mm ky facies are used to establish the cyclostratigraphy of the measured section

characteristics stratigraphic architecture and time framework of multi order mixed siliciclastic and carbonate depositional sequences outcropping cisco group late pennsylvanian and early permian eastern shelf north central texas usa abstract the cisco group on the eastern shelf of the midland basin is composed of fluvial deltaic shelf shelf margin and slope to basin carbonate and siliciclastic rocks sedimentologic and stratigraphic analyses of 181 meter to decimeter scale depositional sequences exposed in the up dip shelf indicated that the siliciclastic and carbonate parasequences in the transgressive systems tracts tst are thin and upward deepening whereas those in highstand systems tracts hst are thick and upward shallowing the sequences can be subdivided into five types on the basis of principal lithofacies and exhibit variable magnitude of facies shift corresponding to variable extents of marine transgression and regression on the shelf the sequence stacking patterns and their regional persistence suggest a three level sequence hierarchy controlled by eustasy whereas local and regional changes in lithology thickness and sequence type magnitude and absence were controlled by interplay of eustasy differential shelf subsidence depositional topography and pattern of siliciclastic supply the outcropping cisco group is highly incomplete with an estimated 6 11 stratigraphic completeness the average duration of deposition of the major third order sequences is estimated as 67 102 ka on the up dip shelf and increases down dip while the average duration of the major sequence boundaries sb is estimated as 831 1066 ka and decreases down dip the nondepositional and erosional hiatus on the up dip shelf was represented by lowstand deltaic systems in the basin and slope

the anisotropy of magnetic susceptibility of lava flows an experimental approach measurements of the anisotropy of magnetic susceptibility ams of natural lavas have shown that ams varies with depth within a lava flow we have investigated the reasons for such variation by studying the effects of temperature and strain rate on the ams of recent lava in the laboratory samples of lava from kilauea were melted and subjected to a range of strain rate and cooling histories the results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample high degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation additionally lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it these results lead to the conclusion that only the last phase of deformation is detectable using ams and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time the relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time consequently flow directions can be inferred confidently with this type of measurements

interbasinal cyclostratigraphic correlation of milankovitch band transgressive regressive cycles correlation of desmoinesian missourian strata between southeastern arizona and the midcontinent of north america the cyclic pattern of deposition in the horquilla limestone of southeastern arizona resulted from glacio eustatic sea level fluctuations with periodicities within the milankovitch band the depositional surface fluctuated from above sea level to below the depth of autochthonous carbonate production by means of characteristics of the cycles correlations can be established not only between sections in the pedregosa basin of southeastern arizona but also to the midcontinent of the united states these correlations exceed the resolution of biostratigraphy and distinguish the eustatic component of relative sea level fluctuations in widely spaced basins with different depositional settings