upper miocene magnetic stratigraphy at odp site 1092 sub antarctic south atlantic recognition of cryptochrons in c5n 2n summary u channel samples from odp site 1092 in the sub antarctic south atlantic ocean provide an interpretable magnetic stratigraphy from 1 95 to 3 6 ma and from 5 9 to 13 5 ma although the interpretation is unclear within the gilbert chron c3 the record represents one of the more complete upper miocene polarity stratigraphies from the south atlantic eight polarity subchrons that are not included in the standard polarity timescale are identified polarity subchron c4r 2r contains a normal polarity interval that has also been identified in sediments from odp leg 138 three short subchrons are recorded within c4n 2n c5r 3r and c5aar the record of c5n 2n contains four short reverse polarity subchrons of durations estimated to be 3 6 kyr

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

high precision zircon u pb geochronology of astronomically dated volcanic ash beds from the mediterranean miocene several orbitally tuned miocene sedimentary sequences around the mediterranean contain abundant intercalated volcanic ash beds these sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology we present a large data set n 16n 16 n 166n 166 of zircon u pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims techniques for ash beds from an almost continuous orbitally tuned messinian to langhian 6 2 15 4 ma sedimentary sequence exposed along the adriatic coast south of ancona italy we use this unique data set to evaluate 1 the accuracy of zircon u pb dates 2 the significance of initial intermediate daughter product disequilibria for zircon u pb geochronology of young rocks 3 the effect of prolonged pre eruption zircon crystallization and zircon recycling on u pb derived ash bed deposition ages and 4 discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the geologic time scale

extending the astronomical polarity time scale into the miocene an astronomical time scale is presented for the late miocene based on the correlation of characteristic sedimentary cycle patterns in marine sections in the mediterranean to the 65 n summer insolation curve of la90 1 2 with present day values for the dynamical ellipticity of the earth and tidd dissipation by the moon this correlation yields ages for all sedimentary cycles and hence also for the recorded polarity reversals and planktonic foraminiferal and dinoflagellate events the tortonian messinian t m boundary placed at the first regular occurrence of the globorofuliu conomiozea group in the mediterranean is dated at 7 24 ma the duration of the messinian is estimated at 1 91 myr because the miocene pliocene boundary has been dated previously at 5 33 ma 3 the new time scale is confirmed by oar 3gar ages of volcanic beds and by the number of sedimentary cycles in the younger part of the mediterranean messinian

geophysical and isotopic constraints on crustal structure related to mineral trends in north central nevada and implications for tectonic history we combined information from sr and pb isotope data and magnetotelluric models to develop a new magnetic and gravity interpretation of the crustal structure of north central nevada to better understand the origin of mineral trends the new interpretation suggests a crustal structure that is composed of precambrian continental crust transitional crust and primarily oceanic crust that are separated by northwest and northeast striking fault zones the magnetic expression of the buried precambrian continental crust is recognized for the first time low magnetic values primarily reflect magnetite poor crystalline crust rather than elevated temperatures at depth northwest and northeast striking crustal boundaries are defined by isotopic data and abrupt gradients in gravity and magnetic data the carlin and battle mountain eureka mineral trends are associated with two of three northwest striking boundaries the carlin boundary is primarily defined by a change in density and isotopic character of the lower to middle crust the battle mountain eureka boundary coincides with a density contrast in the upper crust and a change in isotopic character in the lower to middle crust magnetotelluric models suggest that the battle mountain eureka boundary represents a crustal fault zone for most of its extent but that deep rooted faulting is more complex near and northwest of battle mountain crustal fault zones inferred from the magnetotelluric models near the carlin trend are oblique to it suggesting that they may not have been controlled by the deep boundary seen in the gravity and isotopic data the third northwest trending boundary is related to the western edge of the buried precambrian continent in west central nevada but lacks an associated mineral trend a northeast striking boundary forms the northern limit of precambrian continental and transitional crust the boundaries may have originated as rift or transform faults during precambrian breakup of rodinia or as faults accommodating lateral movements or accretion during later paleozoic tectonic events comparing the crustal structure to tectonic elements produced by successively younger events shows that it had a profound influence on subsequent sedimentation deformation magmatism extension and most important mineralization

cooling mediterranean sea surface temperatures during the late miocene provide a climate context for evolutionary transitions in africa and eurasia abstract in the late miocene grasslands proliferated succulent plants diversified in the mid latitudes and the desert like conditions appeared in the sahara despite this major environmental change on land the coeval deep sea oxygen isotope record does not provide evidence for significant high latitude cooling or continental ice growth making it difficult to relate widespread terrestrial environmental change to global climatic changes a u 37 k derived sea surface temperature sst reconstruction spanning 13 to 6 ma from uplifted hemipelagic sediments in northern italy provides the first continuous mid latitude temperature record with which to compare the evolution of aridity and biotic events at similar latitudes in northern africa and pakistan between 13 and 8 8 ma mediterranean sst lay near the upper limit of the alkenone temperature proxy 28 c exceeding modern sst at the site by as much as 10 c throughout the record sapropel layers correspond to local sst maxima suggesting that late miocene hydrological conditions in the mediterranean responded to insolation forcing via mechanisms similar to those documented for the plio pleistocene mediterranean sst cooled rapidly beginning at 8 ma with an episode of intense cooling to 19 c between 7 2 ma and 6 6 ma followed by a rebound to 25 c preceding the messinian salinity crisis at 5 9 ma these observations establish for the first time a direct relationship between increasing aridity in the northern hemisphere mid latitudes and significant cooling evidently this cooling was not accompanied by significant growth in continental ice volume the extreme warmth and subsequent cooling of the mediterranean sea are not well represented in current late miocene climate models which our results suggest underestimate regional warmth prior to the late miocene cooling evidence of secular cooling during the late miocene gives new support to the much debated link between a possible decline in atmospheric co2 levels and late miocene changes in vegetation on land

age constraints on jerritt canyon and other carlin type gold deposits in the western united states relationship to mid tertiary extension and magmatism carlin type gold deposits are difficult to date and a wide range of ages has been reported for individual deposits therefore several methods were employed to constrain the age of the gold deposits in the jerritt canyon district dated igneous rocks with well documented crosscutting relationships to ore provided the most reliable constraints k ar and 40 ar 39 ar dates on igneous rocks are as follows andesite dikes 324 ma sericitic alteration in andesite dikes 118 ma basalt dikes 40 8 ma quartz monzonite dikes 39 2 ma and calc alkaline ignimbrites 43 1 to 40 1 ma of these only the andesite and basalt dikes are clearly altered and mineralized the gold deposits are therefore younger than the 40 8 ma basalt dikes the sericitic alteration in the andesite dikes is unrelated to the gold deposits a number of dating techniques did not work k ar and 40 ar 39 ar dates on mica from mineralized ordovician to devonian sedimentary rocks gave misleading results the youngest date of 149 ma from the smallest 187 os 188 os initial 0 9 3 6 but did not yield a meaningful isochron due to very low re and os concentrations and large analytical uncertainties paleomagnetic dating techniques failed because the hydrothermal fluids sulfidized nearly all of the iron in the host rocks leaving no remnant magnetism when published isotopic dates from other carlin type deposits in nevada and utah are subject to the rigorous evaluation developed for the jerritt canyon study most deposits can be shown to have formed between 42 and 30 ma k ar and 40 ar 39 ar dates on the youngest preore igneous rocks range from 41 to 32 ma whereas the oldest postore igneous rocks range from 35 to 33 ma hydrothermal adularia from the twin creeks deposit yields similar 40 ar 39 ar dates of 42 ma k ar dates on supergene alunite range from 4 to 30 ma k ar and 40 ar 39 ar dates on micas separated from sedimentary 395 43 ma and igneous 145 38 ma rocks are usually much older than the gold deposits and most are suspect because they are from incompletely reset preore micas or from mixtures of preore and ore stage mica fission track dates on zircons are also generally older than the deposits 169 35 ma and are not completely reset by mineralization apatites are likley to be reset by the hydrothermal systems and by younger thermal events and yield dates 83 22 ma that are younger than those from zircon independent support of a mid tertiary age is provided by the low delta d sub h 2 o values of hydrothermal fluids from 10 widely separated carlin type deposits 134 or 23ppm the low delta d sub h 2 o values are consistent with a mid tertiary age when the climate was cool but are inconsistent with the cretaceous and jurassic ages favored by some workers when the climate was warm the age and distribution of carlin type deposits coincides with areas that underwent extension and calc alkaline magmatism over the same time period despite this relationship the deposits lack clear spatial or genetic relationships to mid tertiary epizonal plutons rather many deposits are localized along preexisting crustal fault zones in the getchell trend structures tapped deep sourced metamorphic or magmatic fluids and subsequently variably exchanged meteoric water in the other trends and districts only variably exchanged meteoric water has been detected these relationships suggest either that all of the deposits formed from deep sourced fluids and were subsequently flooded by meteoric water or that fluids from different sources evolved to produce deposits with similar charateristics

radioisotopic dating of the tortonian global stratotype section and point implications for intercalibration of 40ar 39ar and astronomical dating methods in this paper we present new 40 ar 39 ar data of volcanic ash layers intercalated in the astronomically dated sections of monte dei corvi and monte gibliscemi italy to obtain better radioisotopic time constraints on the serravallian tortonian boundary and to confirm the intercalibration of radioisotopic and astronomical time proposed by kuiper et al 2004 fish canyon tuff fct sanidine at 28 21 0 03 ma the latter intercalibration is supported by astronomically calibrated fct sanidine ages for two ash layers at monte gibliscemi gif 1 28 28 0 04 gid 3 28 16 0 04 ma 1 se as a consequence our results support the astronomically calibrated age of 11 608 ma for the tortonian global stratotype section and point and hence the tuning of the serravallian tortonian boundary interval the ancona and respighi levels at monte dei corvi give a more diffuse picture possibly because of contamination with detrital or xenocrystic material and the inferior quality of biotite for intercalibration

stratigraphic record of the asteroidal veritas breakup in the tortonian monte dei corvi section ancona italy the discovery of elevated concentrations of the cosmogenic radionuclide 3he in deep sea sediments from ocean drilling program odp site 926 atlantic ocean and odp site 757 indian ocean points toward accretion of extraterrestrial matter probably as a result of the catastrophic disruption of a large asteroid that produced the veritas family of asteroids at ca 8 3 0 5 ma and which may have had important effects on the global climatic and ecologic systems here we investigated the signatures possibly related to the veritas event by performing a high resolution multiproxy stratigraphic analysis through the late tortonian early messinian monte dei corvi section near ancona italy closely spaced bulk rock samples through a 36 m thick section approximately spanning from ca 9 9 ma to ca 6 4 ma show an 5 fold 3he anomaly starting at ca 8 5 ma and returning to background values at ca 6 9 ma confirming the global nature of the event we then analyzed at 5 cm intervals bulk rock samples for sedimentary and environmental proxies such as magnetic susceptibility calcium carbonate content total organic carbon and bulk carbonate 18 o and 13 c through a 21 m thick section encompassing the 3he anomaly available high resolution sea surface temperature data via alkenone analyses for this site show a temperature decrease starting exactly at the inception of the 3he anomaly cyclostratigraphic fast fourier transform spectral analyses of the proxies indicate an age of 8 47 0 05 ma for the inception of the 3he anomaly a search for impact ejecta analogous to what is present in the late eocene where both a 3he anomaly and large scale impact events are recorded was not successful detailed cyclostratigraphic analyses of our data suggest that the changes in the stable isotope series and environmental proxy series through this late tortonian time interval had a common forcing agent and that perturbations of orbitally forced climate cycles are present exactly through the interval with the enhanced influx of extraterrestrial 3he thus the chemostratigraphic evidence for a collisional event that created the veritas family of asteroids coinciding with climate perturbations on earth suggests yet another form of interaction between earth and the solar system

evidence for a battle mountain eureka crustal fault zone north central nevada and its relation to neoproterozoic early paleozoic continental breakup 1 combined evidence from gravity radiogenic isotope and magnetotelluric mt data indicates a crustal fault zone that coincides with the northwest trending battle mountain eureka bme mineral trend in north central nevada usa the bme crustal fault zone likely originated during neoproterozoic early paleozoic rifting of the continent and had a large influence on subsequent tectonic events such as emplacement of allochthons and episodic deformation magmatism and mineralization throughout the phanerozoic mt models show the fault zone is about 10 km wide 130 km long and extends from 1 to 5 km below the surface to deep crustal levels isotope data and gravity models imply the fault zone separates crust of fundamentally different character geophysical evidence for such a long lived structure likely inherited from continental breakup defies conventional wisdom that structures this old have been destroyed by cenozoic extensional processes moreover the coincidence with the alignment of mineral deposits supports the assertion by many economic geologists that these alignments are indicators of buried regional structures

completing the neogene geological time scale between 8 5 and 12 5 ma a much improved high resolution magnetostratigraphy is presented for the interval between 8 5 and 12 5 ma in the deep marine astronomically tuned monte dei corvi beach section in northern italy this section contains the formally designated boundary between the middle and upper miocene defined by the tortonian gssp the natural remanent magnetization is carried by an unusual low temperature component of primary origin according to interpretations of isothermal remanent magnetization irm acquisition curves the palaeomagnetic signal is most probably carried by iron sulphides such as greigite or pyrrhotite the resultant magnetostratigraphy is reliable and can be straightforwardly calibrated to the astronomically tuned neogene time scale atnts2004 showing that the section ranges from chron c5an 2n up to c4 2r 1 the inferred correlation of the tortonian gssp to the older part of chron c5r 2n is confirmed thereby guaranteeing global correlation potential in addition the monte dei corvi beach section has been re tuned to the new numerical solution la2004 for the interval between 8 5 and 12 9 ma this exercise shows that the characteristic cycle patterns are not consistent with a single solution with fixed values for dynamical ellipticity and tidal dissipation for the younger part of the section sedimentary cycles show an excellent fit with intricate details of precession obliquity interference in the nominal la2004 1 1 solution however this solution produces serious mismatches between the sedimentary cycles and the precession obliquity interference in the interval older than 10 2 ma for this interval changes in tidal dissipation value while keeping dynamical ellipticity constant at its present day value are necessary to maintain a perfect fit a detailed visual comparison reveals a very good to excellent fit with la2004 1 1 2 and to lesser extent with la2004 1 0 5 however la2004 1 1 2 is the preferred tuning target for the moment in the interval older than 10 2 ma the astronomical re tuning on precessional scale to la2004 1 1 yielded slightly different astronomical ages for each basic cycle and hence also for the calcareous plankton events and magnetic reversal boundaries with uncertainties ranging within a few thousand years the age of the tortonian gssp turns into 11 625 ma instead of the previously published 11 608 ma our ages should replace existing ages in the astronomically tuned neogene time scale atnts2004

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

regional crustal structures and their relationship to the distribution of ore deposits in the western united states based on magnetic and gravity data upgraded gravity and magnetic databases and associated filtered anomaly maps of western united states define regional crustal fractures or faults that may have guided the emplacement of plutonic rocks and large metallic ore deposits fractures igneous intrusions and hydrothermal circulation tend to be localized along boundaries of crustal blocks with geophysical expressions that are enhanced here by wavelength filtering in particular we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large mesozoic and cenozoic ore deposits primarily epithermal and porphyry precious and base metal deposits and sediment hosted gold deposits in the western united states cordillera on the broadest scale most ore deposits lie within areas characterized by low magnetic properties the mesozoic mother lode gold belt displays characteristic geophysical signatures regional gravity high regional low to moderate background magnetic field anomaly and long curvilinear magnetic highs that might serve as an exploration guide geophysical lineaments characterize the idaho montana porphyry belt and the la caridad mineral park belt from northern mexico to western arizona and thus indicate a deep seated control for these mineral belts large metal accumulations represented by the giant bingham porphyry copper and the butte polymetallic vein and porphyry copper systems lie at intersections of several geophysical lineaments at a more local scale geophysical data define deep rooted faults and magmatic zones that correspond to patterns of epithermal precious metal deposits in western and northern nevada of particular interest is an interpreted dense crustal block with a shape that resembles the elliptical deposit pattern partly formed by the carlin trend and the battle mountain eureka mineral belt we support previous studies which on a local scale conclude that structural elements work together to localize mineral deposits within regional zones or belts this study of mineral deposits of the western united states demonstrates the ability of magnetic and gravity data to elucidate the regional geologic framework or structural setting and to contribute in locating favorable environments for hydrothermal mineralization

miocene calibration for calcareous nannofossils from low latitude ocean drilling program sites and the jamaican conundrum prior work with marine sedimentary sections exposed on jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late miocene age we offer an alternative to the jamaican low latitude nannofossil calibration using a combination of paleomagnetic biostratigraphic and lithostratigraphic data derived from tropical sections recovered by the ocean drilling program odp in the eastern pacific and western atlantic oceans our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the pacific ocean and is consistent with the notion that milankovitch periodicities governed cyclic changes in the lithology of sediments from the ceara rise a sea floor high in the atlantic ocean located offshore of the amazon delta our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from jamaican sections

biotic and abiotic response to palaeoenvironmental changes at lake pannons western margin central europe late miocene a multidisciplinary study was performed on a c 30 m thick limnic deltaic sequence in the styrian basin austria geophysical gamma ray activity rock magnetism geochemical organic carbon sulphur and sedimentological observations were combined with palaeontological information mainly ostracods on this base several ecological factors were deduced terrigenous influx salinity water depth and oxygenation based on integrated stratigraphy as well as on palaeomagnetic results the whole section is set to chron c5r 2r 1n 11 308 11 263 ma and covers a period of less than 45 kyr in addition to the long term record we analysed short term changes by high resolution sampling 5 mm sample interval ostracods magnetic susceptibility this 2 3 m thick interval spans in total 3500 yr with an individual sample resolution of a few years only the combination of these data permit the description of the palaeoenvironmental evolution of the section in detail at the base of the section the development of a conifer swamp on the lakeside of lake pannon is documented this almost freshwater swamp existed for some centuries before it became extinct within a few decades due to a rise of the water table the drowning is related to a transgression of lake pannon which triggered the establishment of a brackish water fauna as well as greigite formation in the lake sediments in general the ongoing transgression favoured benthic life due to increased salinity up to mesohaline conditions the high resolution ostracod and magnetic susceptibility record reflect short term fluctuations in bottom water ventilation these oscillations probably range in the order of centuries and decades and are possibly related to climatic shifts later the successive deepening of the lake resulted in a significant faunal turnover a meromictic system with a well established oxygen depleted hypolimnion developed finally the limnic phase was replaced by a prograding deltaic system where the amplified input of coarse grained material and freshwater also affected benthic life periodic changes in clay illite content were detected by the gamma ray log throughout the entire section the observed cycles 5 12 kyr of the gamma ray log may reflect a super ordinate modulation of the sedimentary record by climatically forced changes in precipitation or run off

geologic and geophysical evidence for the influence of deep crustal structures on paleozoic tectonics and the alignment of world class gold deposits north central nevada usa abstract geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the battle mountain eureka bme trend and elsewhere in nevada the fault zones may have originated during proterozoic rifting of the continent and were likely substantially reactivated and modified during paleozoic tectonism five distinct paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified the current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin during the tertiary preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep sourced gold rich fluids which were disseminated into paleozoic slope facies sedimentary rocks forming sediment hosted carlin type and other deposits multiple factors including the locations of these deep seated structures the original configuration of the lower paleozoic continental margin of nevada and its subsequent reactivation during the paleozoic all were fundamental controls on the location of younger mineral deposits a clearer understanding of the original configuration of the margin and of the effects of subsequent paleozoic and mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts

paleomagnetic full vector record of four consecutive mid miocene geomagnetic reversals abstract seventy mid miocene lava flows from flood basalt piles near neskaupstadur east iceland were sampled which provide a quasi continuous record of geomagnetic field variations samples were collected along the profile b of watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 which was extended about 250 m farther down in a neighboring stream bed published radiometric age determinations harrison c mcdougall i watkins n 1979 a geomagnetic field reversal time scale back to 13 0 million years before present earth planet sci lett 42 143 152 range from 12 2 to 12 8 ma for the sampled sequence four reversals were recorded in this profile with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows the large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in icelandic lavas and manifest in a far sidedness of the average vgp the reason for this characteristic which could be related to an anomaly beneath iceland a global field phenomenon local tectonics and or non horizontal flow emplacement is scrutinized non horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano from the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole gad a paleoslope which would explain the observed difference was calculated numerically the obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas the determined paleodip however proved to be significantly too high compared to the usual slope of a central volcano suggesting further reasons for deviations from the gad other datasets of this age from europe also show enhanced vgp dispersion suggesting further contributions of geomagnetic origin for this observation basically all reversal paths move across the pacific transitions were identified as belonging to c5an 1r c5ar 3r based on the astronomically tuned neogene timescale lourens l hilgen f j laskar j shackleton n j wilson d 2004 a geological time scale cambridge university press we selected 122 samples for paleointensity measurements using a modified thellier method including tests for alteration and multidomain bias 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows the average paleointensity for stable field directions was 23 3 t whereas the intensity drops to a minimum of 5 8 t during field transitions the stable field intensities represent only about half of the present day field the saw tooth pattern of intensities which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal was not found in this study

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

40ar 39ar dating of a langhian biotite rich clay layer in the pelagic sequence of the conero riviera ancona italy a nearly complete and undisturbed miocene carbonate sequence is present in the easternmost part of the umbria marche basin italy which is ideal for detailed and integrated stratigraphic investigations of the miocene epoch in this study we were trying to obtain evidence for the presence or absence of distal ejecta from the 15 ma ries impact structure in southern germany located about 600 km to the north northwest of the umbria marche basin the first step is to find coeval strata in the umbria marche sequence at the la vedova section conero riviera we dated a volcaniclastic biotite rich clay layer the aldo level which is situated within planktonic foraminiferal zone n8 at 14 9 0 2 ma using the 40ar 39ar method together with detailed geologic and stratigraphic information about the aldo level the resulting age can be used confidentially to calibrate the langhian stage besides providing new constraints on miocene geochronology this age can now be used for impact stratigraphic studies to directly correlate the biotite ages of the la vedova section with rocks from the ries impact event ries impact glass was also analyzed and found to be coeval although unrelated to this impact event the biotite rich clay layer should help in the search for evidence of distal ejecta related to the ries crater

structural controls on carlin type gold mineralization in the gold bar district eureka county nevada the gold bar district in the southern roberts mountains 48 km northwest of eureka nevada contains one main deposit gold bar five satellite deposits and other resources approximately 0 5 moz of gold have been recovered from a resource of 1 639 000 oz of gold in carlin type gold deposits in lower plate miogeoclinal carbonate rocks below the roberts mountains thrust host rocks are unit 2 of the upper member of the devonian denay formation and the bartine member of the mccolley canyon formation spatial and temporal relations between structures and gold mineralization indicate that both pre tertiary and tertiary structures were important controls on gold mineralization gold mineralization occurs primarily along high angle tertiary normal faults some of which are reactivated reverse faults of paleozoic or mesozoic age most deposits are localized at the intersection of northwest and northeast striking faults alteration includes decalcification and to a lesser extent silicification along high angle faults jasperoid pervasive silicification which formed along most faults and in some strata bound zones accounts for a small portion of the ore in every deposit in the gold canyon deposit a high grade jasperoid pipe formed along a tertiary normal fault which was localized along a zone of overturned fault propagation folds and thrust faults of paleozoic or mesozoic age

an enigma in estimates of the earth s dynamic ellipticity summary the precession and obliquity frequencies of the earth s rotational motion are functions of the dynamic ellipticity of the earth s gravitational figure and this connection has provided a novel bridge between studies of palaeoclimate and geodynamics in particular analyses of tuned climate proxy records have yielded bounds on the mean relative perturbation in dynamic ellipticity over both the last 3 myr and 25 myr that are less than 3 per cent of the non hydrostatic component of the ellipticity we demonstrate that this apparent consistency actually defines an important geophysical enigma over the last 3 myr changes in the earth s figure are likely dominated by ice age forcings in this case a small perturbation to dynamic ellipticity implies significant isostatic compensation of the ice ocean surface mass loads and hence a relatively low mantle viscosity in contrast over the last 25 myr changes in the earth s long wavelength gravitational form are likely dominated by mantle convective flow and in this case the small perturbation to dynamic ellipticity implies sluggish convection and a relatively high mantle viscosity there are at least four possible routes to resolving this enigma the viscosity in the earth s mantle is transient i e dependent on the timescale of the applied forcing tidal dissipation changed in a manner between the last 3 myr and 25 myr that was sufficient to resolve the issue the observationally inferred bounds are unrealistically restrictive or earth models exist in which the ice age and convection effects approximately cancel leading to no net perturbation in this paper we compute a suite of numerical predictions of ice age and convection induced perturbations to the dynamic ellipticity to illustrate the enigma described above

integrated stratigraphy and astrochronology of the messinian gssp at oued akrech atlantic morocco a much improved high resolution integrated stratigraphy calcareous plankton biostratigraphy magnetostratigraphy cyclostratigraphy is presented for the classic section of oued akrech atlantic morocco straddling the tortonian messinian boundary magnetobiostratigraphic correlations with time equivalent and astronomically dated sections in the mediterranean indicate that cyclic alternations of indurated light beige coloured marls and softer more clayey and reddish coloured marls are dominantly precession controlled characteristic sedimentary cycle patterns in particular those reflecting precession obliquity interference allow for one possible tuning thus providing accurate astronomical ages for cycles calcareous plankton events and magnetic reversals the tuning further indicates that the reddish layers are the equivalent of sapropels in the mediterranean the messinian global boundary stratotype section and point gssp has recently been formally defined at the base of the reddish layer of cycle no 15 in section oued akrech this level coincides closely with the first regular occurrence of the globorotalia miotumida group and is astronomically dated at 7 251 ma the global correlation potential is guaranteed by the straightforward calibration of the oued akrech magnetostratigraphy to the geomagnetic polarity time scale locating the gssp within c3br 1r in the marine realm the calcareous nannofossil genus amaurolithus provides a series of extremely useful events to delimit the boundary on a global scale the astronomical tuning guarantees a direct first order calibration of the messinian gssp to the standard geological time scale once as anticipated the late miocene part of the astronomical time scale has been incorporated fl 2000 elsevier science b v all rights reserved

part 1 regional studies and epithermal deposits northern nevada is one of the earth s premier gold and silver producing regions and gold and silver are mined from a wide range of deposit types table 1 current production largely comes from carlin type sedimentary rock hosted disseminated gold deposits christensen 1995 teal and jackson 1997 bettles 2002 many of which lie along linear trends fig 1 roberts 1960 1966 berger and bagby 1991 the small area of the carlin trend approximately 8 x 65 km is north america s most prolific gold mining district producing nearly 125 t 4 moz of gold annually since large scale mining began in 1965 with the opening of the carlin mine the carlin trend has produced more than 1555 t 50 moz of gold during 2001 gold production from the entire state of nevada was nearly 253 t 8 13 moz which accounted for approximately 76 percent of the united states and 10 percent of the world production price and meeuwig 2002 in 2001 only the countries of south africa and australia produced more gold than the state of nevada in addition nevada produced 543 t 17 4 moz of silver in 2001 about 31 percent of the united states production and 3 percent of the world production in 1996 the mineral resources program of the u s geological survey began a multidisciplinary project to investigate the origin of gold deposits in northern nevada this project was undertaken in collaboration with the nevada bureau of mines and geology the ralph j roberts center for research in economic geology at the university of nevada reno several other universities and numerous mining companies active in the region in addition several universities and mining companies were conducting research on the origin of gold deposits in northern nevada this research built on more than a century of studies by the

post deformational growth of late diagenetic greigite in lacustrine sediments from southern italy 1 paleomagnetic rock magnetic and micro textural analyses from a middle pleistocene lacustrine sequence in the southern apennines italy indicate the presence of greigite and magnetite as the main magnetic minerals at different stratigraphic levels in all cases a normal polarity characteristic remanent magnetization chrm was observed in agreement with an 40ar 39ar age of 0 712 0 018 ma from volcaniclastic sanidine after correction for bedding tilt chrm directions carried by greigite do not coincide with the expected geocentric axial dipole field direction at the site latitude whereas the magnetite chrms directions do the data indicate that the greigite magnetization was acquired after tilting and after lock in of the magnetite chrm the estimated delay for the remanence carried by greigite with respect to deposition is 300 ka scanning electron microscope analyses indicate that alteration of detrital volcanic minerals has occurred and that authigenic greigite is generally present in agglomerates and around volcanic grains this observation is consistent with a late diagenetic origin of greigite due to anoxic conditions and availability of dissolved sulfide associated with decomposition of organic matter in the paleolake documentation of a late diagenetic magnetization confirms that care should be taken when using greigite bearing sediments for magnetostratigraphic and tectonic studies

chronology of miocene terrestrial deposits and fossil vertebrates from quebrada honda bolivia abstract at 3500 m above sea level in the eastern andean cordillera quebrada honda basin exposes more than 300 m of middle miocene sedimentary strata developed on distal alluvial fan sediments interstratified with volcanoclastic deposits overlying paleozoic basement these deposits are rich in exotic vertebrate fauna adapted to a wetter and warmer climate at lower paleoelevation as contrasted with the present high elevation cold and dry andean conditions this study provides new magnetostratigraphic and 40ar 39ar age control of the quebrada honda rio rosario and huayllajara fossiliferous areas demonstrating that they are 13 1 12 4 ma part of the succession evidences cyclical deposition of calcretes and reddish mudstones which may be a response to precessional orbital forcing climatic processes paleomagnetic measurements show a significant clockwise tectonic rotation related to postdepositional tectonism associated with evidence of an important uplift after the deposition of quebrada honda sediments this uplift of the andean eastern cordillera was associated with the formation of the subandean fold thrust belt and the development of the amazon and la plata drainage basins proposed uplift rates of 200 m ma are compatible with well established rates of uplift recorded in contemporaneous alpine sections

astrochronology of the mediterranean langhian between 15 29 and 14 17 ma an integrated high resolution magnetobiocyclostratigraphy including radioisotopic dating and astronomical tuning is presented for the interval between 15 29 and 14 17 ma in the marine la vedova section in northern italy the natural remanent magnetization is carried by the iron sulphide greigite and the resultant magnetostratigraphy can be correlated straightforwardly to the interval ranging from c5bn 2n to c5adn in the astronomically tuned neogene time scale atnts2004 spectral analysis on high resolution magnetic susceptibility and geochemical proxy records in the depth domain and using our magnetobiostratigraphic age model in the time domain demonstrate that the various scales of cyclicity in the section are related to astronomical climate forcing starting from our initial age model larger scale cycles were first tuned to eccentricity this first order tuning was followed by tuning the basic cycle to precession and boreal summer insolation using inferred phase relations between maxima in ca al redox sensitive elements and ba and minima in magnetic susceptibility and maxima in precession and minima in obliquity and boreal summer insolation our astronomical ages for reversal boundaries are supported by analysis of sea floor spreading rates and should replace the existing ages in the atnts2004 lacking direct astronomical control two major steps in the geochemical proxy records astronomically dated at 15 074 and 14 489 ma coincide with abrupt changes in sedimentation rate and are the result of the combined effect of the 400 kyr eccentricity cycle superimposed upon a longer term climatic or tectonic induced trend

orbital climate forcing in mudflat to marginal lacustrine deposits in the miocene teruel basin northeast spain abstract upper miocene mudflat and marginal lacustrine sediments were deposited on the fringes of an alluvial fan system in the low gradient endorheic southern teruel basin northeast spain the 147 m thick prado succession consists of 55 meter scale sedimentary cycles displaying a clastic rich and a carbonate rich interval the clastic rich parts are weakly pedogenically modified red to yellow orange mudstones and are interpreted as deposits of a well drained mudflat the carbonate rich sediments consist of calcretes marls and muddy limestones the calcretes range from dispersed nodules to massive nodular and are interpreted to be of pedogenic or phreatic origin the marls and muddy limestones are light to dark gray and rich in organic matter shell fragments and gastropods they are interpreted as ephemeral pond to marginal lacustrine deposits a composite sedimentary cycle model is presented summarizing the different facies and their vertical distribution throughout the prado area on a 10 m scale distinct stratigraphic intervals show lithologic trends representing alternating wetter and drier paleoenvironments cycle characteristics are laterally consistent whereby limestone beds pinch out towards the basin margin and thicken towards the central parts of the 10 km wide and 20 km long subbasin the high resolution magnetostratigraphy resolves four normal polarity and four reversed polarity intervals that are straightforwardly correlated to chron interval c5n 2n to c5ar 1r in the atnts04 time scale the resulting age control indicates a duration of 19 to 23 kyr for the meter scale lithofacies cycles suggesting control by precession driven climate change an astronomical tuning is established by correlating each cycle to the la2004 precession curve using recently recalibrated ages for the polarity reversals as a starting point the resulting astronomical ages for the reversals in the prado section are consistent with the astronomically calibrated magnetostratigraphy and cyclostratigraphy of the marine monte dei corvi section in italy at prado three intervals with thicker than average cycle thickness most likely reflect obliquity influence during 405 kyr eccentricity minima when precession amplitudes are low this corroborates the orbital forcing hypothesis of the lithofacies cycles and the astronomical tuning of the prado succession the combined magnetostratigraphy and cyclostratigraphy method applied in this study shows the high potential of this method for detecting orbital climate forcing in continental successions

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

oligocene miocene basin evolution in se anatolia turkey constraints on the closure of the eastern tethys gateway the oligocene miocene was a time characterized by major climate changes as well as changing plate configurations the middle miocene climate transition 17 to 11 ma may even have been triggered by a plate tectonic event the closure of the eastern tethys gateway the marine connection between the mediterranean and indian ocean to address this idea we focus on the evolution of oligocene and miocene foreland basins in the southernmost part of turkey the most likely candidates to have formed this gateway in addition we take the geodynamic evol ution of the arabian eurasian collision into account the musand elazog u basins located to the north of the bitlis zagros suture zone were most likely connected during the oligocene the deepening of both basins is biostratigraphically dated by us to occur during the rupelian early oligocene deep marine conditions between 350 and 750 m prevailed until the chattian late oligocene when the basins shoaled rapidly to subtidal intertidal environment in tropical to subtropical conditions as indicated by the macro fossil assemblages we conclude that the emergence of this basin during the chattian severely restricted the marine connection between an eastern indian ocean and western mediterranean marine domain if a connection persisted it was likely located south of the bitlis zagros suture zone the kahramanmarasbasin located on the northern arabian promontory south of the bitlis zagros suture zone was a foreland basin during the middle and late miocene possibly linked to the hatay basin to the west and the lice basin to the east our data indicates that this fore land basin experienced shallow marine conditions during the langhian followed by a rapid dee pening during langhian serravallian and prevailing deep marine conditions between 350 and 750 m until the early tortonian we have dated the youngest sediments underneath a subduc tion related thrust at c 11 ma and suggest that this corresponds to the end of underthrusting in the kahramanmarasregion i e the end of subduction of arabia this age coincides in time with the onset of eastern anatolian volcanism uplift of the east anatolian accretionary complex and the onset of the north and east anatolian fault zones accommodating westward escape tec tonics of anatolia after c 11 ma the foreland basin south of the bitlis formed not or no longer a deep marine connection along the northern margin of arabia between the mediterranean sea and the indian ocean we finally conclude that a causal link between gateway closure and global climate change to a cooler mode recorded in the mi3b event d 18 o increase dated at 13 82 ma cannot be supported

crustal structure of the ruby mountains and southern carlin trend region nevada from magnetotelluric data abstract we have collected about 150 magnetotelluric mt soundings in northeastern nevada in the region of the ruby mountains metamorphic core complex uplift and southern carlin mineral trend in an effort to illuminate controls on core complex evolution and deposition of world class gold deposits the region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation which have contributed to the total expression of electrical resistivity most of the soundings reside in three east west profiles across increasing degrees of core uplift to the north bald mountain harrison pass and secret pass latitudes one short cross line was also taken to assess an east west structure to the north of the northern profile model resistivity cross sections were derived from the mt data using a 2 d inversion algorithm which damps departures of model parameters from an a priori structure geological interpretation of the resistivity combines previous seismic potential field and isotope models structural and petrological models for regional compression and extension and detailed structural stratigraphic interpretations incorporating drilling for petroleum and mineral exploration to first order the resistivity structure is one of a moderately conductive phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks late devonian and early mississippian shales of the pilot and chainman formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance conductivity thickness product from east to west these increases are attributed to graphitization caused by elko sevier era compressional shear deformation and possibly by intrusive heating the resistive crystalline central massifs adjoin the host stratigraphy across crustal scale steeply dipping fault zones the zones provide pathways to the lower crust for heterogeneous upper crustal induced electric current flow resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west the numerous crustal breaks imaged with mt may contribute to the low effective elastic thickness t e estimated regionally for the great basin and exemplify the mid crustal steeply dipping slip zones in which major earthquakes nucleate an east west oriented conductor in the crystalline upper crust spans the east humboldt range and northern ruby mountains the conductor may be related to nearby graphitic metasediments with possible alteration by middle tertiary magmatism lower crustal resistivity everywhere under the profiles is low and appears quasi one dimensional it is consistent with a low rock porosity

cyclostratigraphic analysis of pelagic carbonates at monte dei corvi ancona italy and astronomical correlation of the serravallian tortonian boundary cyclostratigraphic analysis of pelagic carbonates at monte dei corvi ancona italy and astronomical correlation of the serravallian tortonian boundary spectral analysis of high resolution calcium carbonate data from a 2 m y long section from the upper miocene pelagic limestone marl sequence at monte dei corvi italy indicates orbital forcing of depositional cycles at frequencies corresponding to eccentricity obliquity and precessional cycles identification of these milankovitch cycles dated by 4 0 ar 3 9 ar ages from two biotite rich volcanic ash layers within the section allows us to correlate this section with the astronomical time scale by matching low frequency variations in calcium carbonate content with the orbital eccentricity this correlation yields precise numerical ages for any stratigraphic level within the analyzed section including the two volcanic ash layers and the serravallian tortonian boundary dated here as 11 32 ma the eccentricity based correlation also allows us to tune the data resulting in enhanced power in the obliquity and precessional frequencies supporting the initial hypothesis of orbital control of these lithologic cycles

mediterranean neogene planktonic foraminifer biozonation and biochronology abstract planktonic foraminifera are widely used for biostratigraphy and correlation of mediterranean neogene marine sediments and are a fundamental component in the astronomical tuning of the neogene time scale recent developments in high resolution studies focused on the astronomical calibration of cyclically marine sediments cropping out in land based sections and recovered from deep sea successions increased the accuracy of stratigraphic ranges of planktonic foraminiferal species improving the biostratigraphic resolution and biochronology the large amount of data on planktonic foraminifera obtained through quantitative semiquantitative analyses published in the recent years allowed the revision of many biohorizons and their calibrations we incorporate these developments and emendments into the existing mediterranean planktonic foraminiferal biozonation therefore in this paper we present an emended standard mediterranean planktonic foraminiferal biozonation with a detailed description of zones and subzones within the framework of the astronomical tuned neogene time scale 2004 and 2012 atnts2004 atnts2012 and we provide the range chart of the most common planktonic foraminiferal taxa and the quantitative distribution pattern of selected marker species twenty two biozones and thirty one subzones that span the past 23 million years have been identified we distinguished them using the following code system mmi1 to mmi13 and ndz for the time interval related to the messinian salinity crisis mediterranean miocene biozones mpl1 to mpl6 mediterranean pliocene biozones according to the gelasian as the uppermost stage of the pliocene series epoch and mple1 to mple2 mediterranean pleistocene biozones according to the calabrian as the lowermost stage of the pleistocene series epoch we assembled 118 neogene planktonic foraminiferal biohorizons from multiple datasets and incorporated the calibration of these bioevents into a revised neogene planktonic foraminiferal biochronology the revised and recalibrated data provide a major progress in biostratigraphic and biochronologic resolution and a template for future progress of the neogene time scale unfortunately two main gaps of planktonic foraminiferal quantitative data occur in the late burdigalian between 16 12 ma and 17 23 ma and at aquitanian burdigalian boundary between 19 74 ma and 20 66 ma due to the absence of high resolution studies of these time intervals in the mediterranean

cyclostratigraphy of an orbitally driven tithonian valanginian carbonate ramp succession southern mendoza argentina implications for the jurassic cretaceous boundary in the neuquen basin we conducted detailed sedimentological sequence stratigraphic and cyclostratigraphic analyses on four sections of the vaca muerta formation this unit is characterized by a decimetre scale rhythmic alternation of marls and limestones it consists of five facies associations which represent basin to middle carbonate ramp deposits the rhythmic vertical organization shows a well ordered hierarchy of cycles where elementary cycles bundles of cycles and superbundles within the milankovitch frequency band are recognized cyclostratigraphic data allowed us to build a floating orbital scale for the lower tithonian lower valanginian interval in the neuquen basin orbital calibration of these sections is consistent with riccardi s biostratigraphic scheme which places the jurassic cretaceous boundary within the substeueroceras koeneni ammonite zone equivalent to durangites spp jacobi grandis and initial occitanica standard zones

geological constraints on tidal dissipation and dynamical ellipticity of the earth over the past three million years the evolution of the solar system has been shown to be chaotic which limits our ability to retrace the orbital and precessional motion of the earth over more than 35 50 myr ref 2 moreover the precession obliquity and insolation parameters can also be influenced by secular variations in the tidal dissipation and dynamical ellipticity of the earth induced by glacial cyclicity and mantle convection here we determine the average values of these dissipative effects over the past three million years we have computed the optimal fit between an exceptional palaeoclimate record from the eastern mediterranean sea and a model of the astronomical and insolation history by testing a number of values for the tidal dissipation and dynamical ellipticity parameters we find that the combined effects of dynamical ellipticity and tidal dissipation were on average significantly lower over the past three million years compared to their present day values determined from artificial satellite data and lunar ranging this secular variation associated with the plio pleistocene ice load history has caused an average acceleration in the earth s rotation over the past 3 myr which needs to be considered in the construction of astronomical timescales and in research into the stationarity of phase relations in the ocean climate system through time

odp site 1092 revised composite depth section has implications for upper miocene cryptochrons summary an upper miocene magnetic polarity stratigraphy has been developed for the c3r to c5abn interval of odp site 1092 from the sub antarctic south atlantic evans channell 2003 new data from x ray fluorescence xrf scanning of half cores has led to development of a revised composite depth section for the site this has implications for the magnetostratigraphic interpretation using the shipboard derived composite section four subchrons were identified within the normal polarity zone correlative to c5n 2n the revised composite section results in subchrons c5n 2n 2 and c5n 2n 3 of evans channell 2003 becoming a single subchron recorded in two overlapping cores the revised number of polarity subchrons within c5n 2n is now three rather than four which is consistent with the number of cryptochrons recognized in marine oceanic anomaly data

high frequency cyclicity in a miocene sequence of the vienna basin established from high resolution logs and robust chronostratigraphic tuning abstract a high resolution chronostratigraphic record established by w e paulissen s m luthi p grunert s coric m harzhauser geol carp 2011 that takes into account variations in sedimentation rates and temporal gaps caused by unconformities and faulting in a research borehole in the vienna basin was used to investigate the possible presence of orbital millennial and centennial periodicities the sedimentary sequence covers middle to late miocene shallow marine fluvio deltaic and lacustrine shales silt and sandstones deposited during the transition from a pull apart basin to the final infill in a compressional regime spectral analysis was performed on gamma ray logs and high resolution electrical borehole images using three different analytical methods over six suitable intervals where continuous and constant sedimentation was identified the significant periods were found to closely match the orbital cycles of precession obliquity and short eccentricity providing a solid basis for the analysis of potential sub milankovitch cycles the significant frequencies encountered at the millennial to centennial scale fell for 75 within a relatively narrow time period of 0 25 to 5 kyr with a concentration of peaks between 1 and 2 kyr 29 and 500 to 800 years 17 these periodicities relate closely to the millennial dansgaard oeschger and bond cycles and centennial scale climate cycles documented from the quaternary it is suggested that the high frequency cycles observed in the miocene of the vienna basin represent differences in grain size related to cyclic variations in regional precipitation

relationship of epithermal gold deposits to large scale fractures in northern nevada geophysical maps of northern nevada reveal at least three and possibly six large scale arcuate features one of which corresponds to the northern nevada rift that possibly extends more than 1 000 km from the oregonidaho border to southern nevada these features may reflect deep discontinuities within the earth s crust possibly related to the impact of the yellowstone hot spot because mid miocene epithermal gold deposits have been shown to correlate with the northern nevada rift we investigate the association of other epithermal gold deposits to other similar arcuate features in northern nevada mid miocene and younger epithermal goldsilver deposits also occur along two prominent aeromagnetic anomalies west of the northern nevada rift here we speculate that mid miocene deposits formed along deep fractures in association with mid miocene riftrelated magmatism and that younger deposits preferentially followed these preexisting features statistical analysis of the proximity of epithermal gold deposits to these features suggests that epithermal gold deposits in northern nevada are spatially associated with large scale crustal features interpreted from geophysical data

shallow lacustrine carbonate microfacies document orbitally paced lake level history in the miocene teruel basin north east spain results are presented of a detailed carbonate petrographic study of an upper miocene lacustrine mixed carbonate siliciclastic succession in the teruel basin spain with the aim of constraining lake level variability at different stratigraphic scales regular alternations of red to green mudstone and lacustrine limestone termed the basic cycle reflect lake level variations at the metre scale in an earlier study the basic cycle was shown to be controlled by the climatic precession cycle petrographic analysis made it possible to distinguish two main carbonate microfacies groups characteristic of very shallow transient and shallow permanent lake environments respectively in addition to the basic cyclicity the microfacies analysis reveals lake level variations on a larger scale as a consequence the astronomical forcing hypothesis of the cyclicity in the cascante section is explored further a climate modelling study of orbital extremes indicates that high lake levels could relate to enhanced net winter precipitation and runoff during precession minima consistent with mediterranean geological data using this phase relationship an astronomical tuning of the cycles is established starting from astronomical ages of magnetic reversal boundaries subsequently successive basic cycles are correlated to precession minima the tuning reveals an identical number of basic cycles in the cascante section as precession related sapropel cycles in the deep marine succession at monte dei corvi italy corroborating the precessional control of the basic cycles at cascante lake level highstands in the large scale cycle identified by the microfacies analysis relate to maxima in both the ca 100 and 405kyr eccentricity cycles again consistent with mediterranean geological data subtraction of the identified astronomically related lake level variations from the palaeoenvironmental record at cascante indicates a shift to deeper and more permanent lacustrine environments in the upper half of the section the cause of this shift remains unclear but it may be linked to tectonics non astronomical climate long period astronomical cycles or autogenic processes 2008 the authors journal compilation 2008 international association of sedimentologists

astronomical tuning of the la vedova high cliff section ancona italy implications of the middle miocene climate transition for mediterranean sapropel formation article i nfo continuous marine successions covering the middle miocene climate transition mmct 15 13 7 ma are scarce and the lack of a high resolution magnetobiostratigraphic framework hampers the construction of astronomically tuned age models for this time interval the la vedova high cliff section exposed along the coast of the conero riviera near ancona italy is one of the few mediterranean sections covering the critical time interval of the mmct starting from an initial magnetobiostratigraphic age model a robust astronomical tuning was constructed for the interval between 14 2 and 13 5 ma using geochemical element data and time series analysis a shift in 18 o of bulk sediment towards heavier values occurs between 13 92 and 13 78 ma and could be related to the mi3b oxygen isotope event which reflects the rapid expansion of the east antarctic ice sheet in the middle miocene the onset of the cm6 carbon excursion is reflected in the bulk record by a rapid increase in 13 c at 13 86 ma our results confirm the proposition that these events coincide with a 405 kyr minimum in eccentricity and a node in obliquity related to the 1 2 myr cycle from 13 8 ma onwards distinct quadruplet cycles containing sapropelitic sediments were deposited this may suggest a causal connection between the main middle miocene cooling step and the onset of sapropel formation in the

time scale controversy accurate orbital calibration of the early paleogene 1 timing is crucial to understanding the causes and consequences of events in earth history the calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating uncertainties in the computations of earth s orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 ma attempts to construct a robust astronomically tuned time scale for the early paleogene by integrating radioisotopic and astronomical dating are only partially consistent here using the new la2010 and la2011 orbital solutions we present the first accurate astronomically calibrated time scale for the early paleogene 47 65 ma uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the fish canyon standard comparison with geological data confirms the stability of the new la2011 solution back to 54 ma subsequent anchoring of floating chronologies to the la2011 solution using the very long eccentricity nodes provides an absolute age of 55 530 0 05 ma for the onset of the paleocene eocene thermal maximum petm 54 850 0 05 ma for the early eocene ash 17 and 65 250 0 06 ma for the k pg boundary the new astrochronology presented here indicates that the intercalibration and synchronization of u pb and 40ar 39ar radioisotopic geochronology is much more challenging than previously thought

testing the relationship between timing of geomagnetic reversals excursions and phase of orbital cycles using circular statistics and monte carlo simulations abstract fuller fuller m geomagnetic field intensity excursions reversals and the 41 000 yr obliquity signal earth planet sci lett 245 2006 605 615 pointed out that for 9 reversals over the last 3 myr reversal age has a non random relationship to the phase of orbital obliquity our analysis based on rayleigh tests indicates that reversals have no preferred phase distribution in the obliquity cycle at the 5 significance level over the last 3 myr there is however a statistically significant relationship at the 5 level between reversal age and the phase of orbital eccentricity for the last 3 myr although this relationship breaks down on adding just a few reversals beyond 3 ma over the last 5 myr reversals preferentially occurred during decrease of the maximum obliquity envelope although yet again the relationship does not hold as additional reversals are added to the analysis no matter which timescale is tested the rayleigh tests are all based on the assumption of no uncertainty in reversal excursion age or in orbital solutions monte carlo simulations indicate that reversal excursion ages would have to be known within 5 10 kyr to resolve a preferred phase in obliquity similar to that advocated by fuller fuller m geomagnetic field intensity excursions reversals and the 41 000 yr obliquity signal earth planet sci lett 245 2006 605 615 over the last 3 myr reversal excursion ages would have to be known within 15 kyr to resolve a preferred phase in orbital eccentricity for reversals over the last 3 myr and within 40 kyr for the last 25 myr comparison of astrochronological reversal timescales indicates that reversal age uncertainties exceed these limits making it unlikely that a relationship of reversal excursion age to the phase of obliquity or eccentricity would be resolvable in the case of the obliquity envelope the critical levels of reversal age uncertainty 50 kyr for 0 3 ma 200 kyr for 0 5 ma and 400 kyr for 0 25 ma are less stringent the presence of a significant relationship between reversal age and phase of the obliquity envelope for the last 5 myr but not further back in time implies either larger than expected reversal age uncertainties in pre pliocene polarity timescales and a link between reversal age and the obliquity envelope or more probably the fortuitous occurrence of a low probability relationship over the last 5 ma that has no mechanistic implication

integrated stratigraphy and astronomical calibration of the serravallian tortonian boundary section at monte gibliscemi sicily italy results are presented of an integrated stratigraphic calcareous plankton biostratigraphy cyclostratigraphy and magnetostratigraphy study of the serravallian tortonian s t boundary section of monte gibliscemi sicily italy astronomical calibration of the sedimentary cycles provides absolute ages for calcareous plankton bio events in the interval between 9 8 and 12 1 ma the first occurrence fo ofneogloboquadrina acostaensis usually taken to delimit the s t boundary is dated astronomically at 11 781 ma pre dating the migratory arrival of the species at low latitudes in the atlantic by almost 2 million years in contrast to delayed low latitude arrival of n acostaensis paragloborotalia mayeri shows a delayed low latitude extinction of slightly more than 0 7 million years with respect to the mediterranean last occurrence lo at 10 49 ma at ceara rise lo at 11 205 ma in the mediterranean the discoaster hamatus fo dated at 10 150 ma is clearly delayed with respect to the open ocean the ages of d kugleri first and last common occurrence fco and lco catinaster coalitus fo coccolithus miopelagicus last regular occurrence l r o and the d hamatus neohamatus cross over however are in good to excellent agreement with astronomically tuned ages for the same events at ceara rise tropical atlantic suggesting that both independently established timescales are consistent with one another the lack of a reliable magnetostratigraphy hampers a direct comparison with the geomagnetic polarity timescale of cande and kent 1995 ck95 but ages of calcareous nannofossil events suggests that ck95 is significantly younger over the studied time interval approximate astronomical ages for the polarity reversals were obtained by exporting astronomical ages of selected nannofossil events from ceara rise and the mediterranean to eastern equatorial pacific odp leg 138 site 845 which has a reliable magnetostratigraphy our data from the rio mazzapiedi castellania section reveal that the base of the tortonian stratotype corresponds almost exactly with the first regular occurrence fro of n acostaensis s s as defined in the present study dated at 10 554 ma an extrapolated age of 11 8 ma calculated for the top of the serravallian stratotype indicates that there is a gap between the top of the serravallian and the base of the tortonian stratotype potentially rendering all bio events in the interval between 11 8 and 10 554 ma suitable for delimiting the s t boundary despite the tectonic deformation and the lack of a magnetostratigraphy gibliscemi remains a candidate to define the s t boundary by means of the tortonian global boundary stratotype section and point gssp 2000 elsevier science b v all rights reserved

magnetostratigraphy and radio isotope dating of upper miocene lower pliocene sedimentary successions of the black sea basin taman peninsula russia abstract we present a new chronology for the upper miocene to pliocene deposits of the black sea basin based on high resolution magnetostratigraphic data coupled with 40 ar 39 ar dating from the 475 m long zheleznyi rog section on the taman peninsula russia this section comprises the stratigraphic interval of the khersonian to kimmerian regional stages of the eastern paratethys our magnetostratigraphic record is based on biogenic and early diagenetic greigite components and yields a magnetostratigraphic sequence of 8 polarity zones a volcaniclastic ash layer at the upper part of the khersonian is radio isotopically dated at 8 69 0 18 ma the age of the khersonian meotian transition arrives at 8 6 or 8 2 ma which is significantly younger than earlier estimates the new age has important consequences for the hipparion datum in the eastern paratethys and implies that all vallesian fossil sites are younger than 11 2 ma the meotian stage has a total duration of 2 2 to maximum 2 6 myr and is marked by multiple erosional events the end of the paratethys sea is dated at 5 5 ma when the black sea becomes isolated from the caspian sea the dacian basin and the mediterranean

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

an astronomical polarity timescale for the late middle miocene based on cyclic continental sequences 1 we present an astronomically tuned polarity timescale for the late middle miocene based on a cyclic shallow lacustrine mudflat succession exposed in the orera composite section ocs calatayud basin ne spain spectral analysis and band pass filtering of high resolution carbonate and color reflectance records in the depth domain reveal cyclic changes with different cycle lengths which correspond to lithological alternations observed in the field an initial age model was constructed by calibrating the ocs magnetostratigraphy to the geomagnetic polarity timescale of cande and kent 1995 subsequent spectral analysis of the proxy records in the time domain reveals periodicities close to 23 41 and 400 kyr and to a lesser extent 100 kyr supporting an astronomical origin for the sedimentary cyclicity in the ocs we established a new age model based on the astronomical calibration of the ocs to the laskar et al 1993 la93 solution by tuning the cycles to the astronomical target curves cross spectral analysis results of the tuned time series followed by band pass filtering reveal a remarkably good and in phase relation with precession and with obliquity despite a presumed uncertainty of 20 40 kyr in the tuning in some short intervals our tuning provides astronomical ages for sedimentary cycles and subsequently for polarity reversals in the interval between 12 9 and 10 6 ma comparison with cande and kent shows that the ocs polarity reversal ages are older by 80 kyr this age discrepancy decreases with increasing age to 50 60 kyr index terms 1035 geochemistry geochronology 1520 geomagnetism and paleomagnetism magnetostratigraphy 8105 tectonophysics continental margins and sedimentary basins 9335 information related to geographic region europe 9604 information related to geologic time cenozoic keywords geochronology magnetostratigraphy cyclostratigraphy astronomical timescale orbital forcing miocene

integrated stratigraphy and astronomical tuning of the serravallian and lower tortonian at monte dei corvi middle upper miocene northern italy an integrated stratigraphy calcareous plankton biostratigraphy magnetostratigraphy and cyclostratigraphy is presented for the serravallian and lower tortonian part middle upper miocene of the monte dei corvi section located in northern italy the detailed biostratigraphic analysis showed that both the discoaster kugleri acme and the first influx of neogloboquadrina acostaensis are recorded at monte dei corvi these events which passed unobserved in previous studies play an important role in delineating the serravallian tortonian boundary thermal and alternating field demagnetization revealed a characteristic low temperature component marked by dual polarities the resultant magnetostratigraphy for the upper part of the section can be unambiguously calibrated to the gpts ranging from c5n 2n up to c4r 2r unfortunately the lower part of the section including the serravallian tortonian boundary interval did not produce a reliable magnetostratigraphy despite the fact that some short reversed intervals and a single normal interval are recorded using sedimentary cycle patterns in combination with the calcareous plankton biostratigraphy the section can be correlated cyclostratigraphically in detail to the partially overlapping and previously tuned section of monte gibliscemi on sicily the monte dei corvi section is dated astronomically by calibrating the basic small scale sedimentary cycles to the precession and 65 n lat summer insolation time series of the la93 solution following an initial tuning of larger scale cycles to eccentricity an almost perfect fit is found between the cycle patterns and intricate details especially precession obliquity interference in the insolation target between 8 5 and 10 ma the tuning to precession remains robust for most intervals back to the base of the section dated at 13 4 ma and shows that the section is continuous apart from a possible short hiatus in the tortonian it provides accurate astronomical ages for all sedimentary cycles calcareous plankton events polarity reversals and ash layers and marks a significant improvement of the recently proposed astronomical calibrations of the monte dei corvi section and of parallel sections in the mediterranean astronomical ages for the ancona and respighi ashbeds are significantly older than previously reported 40 ar 39 ar biotite ages even if the revised older age for the fct san dating standard of 28 02 ma is applied the astronomical dating of the magnetic reversals in the monte dei corvi section results in the completion of the astronomical polarity time scale for the last 13 myr the monte dei corvi section has recently been proposed as the stratotype section for the serravallian tortonian boundary despite the moderate to

orbitally paced climate evolution during the middle miocene monterey carbon isotope excursion one of the most enigmatic features of cenozoic long term climate evolution is the long lasting positive carbon isotope excursion or monterey excursion which started during a period of global warmth after 16 9 ma and ended at similar to 13 5 ma approximately 400 kyr after major expansion of the antarctic ice sheet we present high resolution 1 9 kyr astronomically tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern pacific odp sites 1146 and 1237 which shed new light on the middle miocene carbon isotope excursion and associated climatic transition over the interval 17 1 12 7 ma we recog nize three distinct climate phases with different imprints of orbital variations into the climatic signals 1146 and 123 7 delta 18 o delta 13 c 1237 xrt fe fraction 63 pm 1 climate optimum prior to 14 7 ma characterized by minimum ice volume and prominent 100 and 400 kyr variability 2 long term cooling from 14 7 to 13 9 ma principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced delta 13 c increase 3 icehouse mode after 13 9 ma with distinct 100 kyr variability and improved ventilation of the deep pacific the monterey carbon isotope excursion 16 9 13 5 ma consists overall of nine 400 kyr cycles which show high coherence with the long eccentricity period superposed on these low frequency oscillations are high frequency variations 100 kyr which closely track the amplitude modulation of the short eccentricity period in contrast to delta 13 c the delta 18 o signal additionally shows significant power in the 41 kyr band and the 1 2 myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 delta 18 o signal our results suggest that eccentricity was a prime pacemaker of middle miocene climate evolution through the modulation of long term carbon budgets and that obliquity paced changes in high latitude seasonality favored the transition into the icehouse climate c 2007 elsevier b v all rights reserved