high resolution late maastrichtian early danian oceanic 87sr 86sr record implications for cretaceous tertiary boundary events a high resolution late maastrichtian early danian seawater 87 sr 86 sr reference curve is constructed from two cretaceous tertiary boundary k t boundary sections bidart france and el kef tunisia the 87 sr 86 sr curve shows maxima at 0 3 0 4 ma before the k t boundary and at the k t boundary the first maximum could mark the onset of a major outflow of the deccan traps the second maximum a rapid 0 000 06 87 sr 86 sr shift extends from 3 4 m below to 1 m above the k t boundary this profile probably results from diagenetic smoothing of an originally sharp k t boundary 87 sr 86 sr anomaly rather than from a gradual process the sharp shift could result from 1 the vaporization of the chicxulub target rocks 2 global wildfires and 3 acid rain leaching of soils and sialic surface rocks of these three possibilities only sr release by soil leaching combined with increased rainfall associated with the k t event appears to be sufficiently large to produce the observed k t 87 sr 86 sr anomaly

the cretaceous tertiary boundary stratotype section at el kef tunisia how catastrophic was the mass extinction abstract the cretaceous tertiary k t boundary stratotype section at el kef tunisia represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known high resolution analysis of planktic foraminifera in two outcrops el kef i stratotype and el kef ii along with comparisons between planktic and benthic foraminifera calcareous nannofossils ostracods pollen and spores and dinoflagellates indicate that major changes across the k t boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils biotic changes in benthic foraminifera are unique to el kef and similarly shallow continental shelf sections and appear to be the result of a sea level regression in the latest maastrichtian followed by a sea level rise across the k t boundary that was accompanied by expansion of the local oxygen minimum zone omz biotic changes in planktic foraminifera appear partly related to these conditions also but in general reflect more global oceanographic changes for instance species extinctions are gradual and selective as observed in k t sections worldwide rather than random and abrupt although there is a 69 decline in species richness between 25 cm below and 10 cm above the k t boundary only rare species disappeared their combined relative abundance constitute less than 20 of the total population about 52 of these extinct taxa 8 of the population are large ornate morphologically complex tropical subtropical forms that lived at or below the thermocline no planktic foraminifera from this depth range survived the k t boundary event all survivor taxa were surface dwellers living within the photic zone their relative abundance 80 dominates both cretaceous and early tertiary populations these data indicate that the k t biotic record in the shallow continental shelf section at el kef was significantly influenced by local conditions which combined with the latest maastrichtian sea level regression and subsequent sea level rise resulted in shallowing of the local omz relative to the sea surface shallowing of the local omz lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers the selective nature of species extinctions however appear to be related partly to long term global oceanographic changes which were accelerated at the k t boundary possibly by a bolide impact

seismotectonics of the san andreas fault system between point arena and cape mendocino in northern california implications for the development and evolution of a young transform the northernmost and relatively youthful segment of the san andreas fault system is situated within a 100 km wide zone of northwest trending strike slip faults that includes from west to east the san andreas maacama and bartlett springs faults although the san andreas fault is the principal strike slip fault in this system it has been virtually aseismic since the 1906 earthquake moderate levels of seismicity locate to the east along the maacama fault and to a lesser extent the bartlett springs fault at focal depths typical of other strike slip faults within the san andreas fault system in central california north of the san andreas fault system within the cape mendocino area earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting gorda slab and slip along the mendocino fracture zone seismicity along the maacama and bartlett springs faults is dominated by right lateral to oblique reverse slip along fault planes that dip 50 75 to the northeast the northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the gorda slab the onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the north american plate as it enters the asthenospheric window the maacama and bartlett springs faults are strike parallel with active reverse faults within the forearc region of the cascadia subduction zone this preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults continuation of the moderately dipping maacama fault to the southeast along the steeply dipping healdsburg and rodgers creek fault zones and the near vertical hayward and calaveras fault zones in the san francisco bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion

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

rapid change in strontium isotopic composition of sea water before the cretaceous tertiary boundary a critical aspect of the debate over the origin of the chemical biological and climatological perturbations that characterize the cretaceous tertiary boundary k t boundary is the timescale and detailed pattern of the extinctions and geochemical changes whether the development of the anomalies was catastrophic over tens to hundreds of years or more gradual 103 to 106 years whether the anomalies occur exactly at the boundary or precede it and whether the chemical and biological signatures represent one event or can be resolved into several discrete events are issues fundamental to identifying the cause of the marked changes to determine the timing and rate of change of strontium isotope evolution of sea water during the five million years preceding the k t boundary we analysed foraminifera from an exceptionally thick palaeontologically well characterized marine k t section exposed at bidart southwest france we found a rapid increase in 87sr 86sr of ocean water 1 5 to 2 3 myr before the boundary the increase may be explained by a 10 increase in continental strontium flux to the oceans over a period of about 1 myr

an iridium rich layer at the cretaceous tertiary boundary in the bidart section southern france we have measured an anomalous iridium concentration in the clays of the cretaceous tertiary boundary in the bidart section thus confirming the planetary character of this anomaly the iridium anomaly is synchronous with a very sharp minimum of the 13c 12c ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary the integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the pacific ocean in spite of a careful search we have found no evidence of any exotic material

the 1936 1945 1947 and 1950 earthquake sequences near lassen peak california three vigorous earthquake sequences occurred near lassen peak in 1936 between 1945 and 1947 and in 1950 the latter two sequences included mainshocks of magnitude 5 0 and 5 5 respectively and thousands of smaller events no comparable earthquake sequences have occurred near lassen peak since 1950 the epicentral area lies within 20 km of the southern boundary of lassen volcanic national park in a northwest striking seismic zone that extends from lake tahoe to the vicinity of mount shasta in comparing their time history and magnitude distribution with other earthquake sequences that have occurred in regions of cenozoic volcanism within and east of the cascade range and the sierra nevada we find that the lassen earthquake sequences show similar characteristics to two earthquake sequences that occurred on basin and range faults near herlong california and klamath falls oregon we interpret this similarity as evidence that the lassen earthquakes were caused by basin and range extension and may have occurred on one or more basin and range faults in the lassen region however the limitations of the data do not allow other possible sources such as magmatic injection to be ruled out the most important implication of the lassen earthquake sequences is that earthquakes of m 5 or greater may occur in the lassen region perhaps quite close to lassen peak or other volcanoes the record of holocene volcanism and fault displacements in the region indicates that earthquake sequences driven by either tectonic or magmatic processes may occur near lassen peak and any significant earthquake sequence should be carefully monitored to assess its nature

high pressure low temperature metamorphism and deformation in the bundnerschiefer of the engadine window implications for the regional evolution of the eastern central alps a study of the metamorphic and tectonic evolution of the bundnerschiefer of the engadine window shows that the individual nappes have been thinned by a large amount and that extension was active during and soon after nappe stacking based on contrasting p t histories the penninic bundnerschiefer can be divided in two major units bounded by a horizontal contact the lower mundin unit shows typical high p low t parageneses in metapelites mg carpholite and in metabasites glaucophane metamorphic conditions are estimated around 12 kbar 375 c the upper arina unit contains no specific high p minerals metamorphic conditions are estimated around 7 kbar 325 c a minimum pressure gap of 5 kbar is thus observed the contact between the two units is marked by a mappable normal shear zone with top to the north west sense of shear near the shear zone fresh carpholite fibres trend parallel to the regional stretching lineation implying that the detachment is an early structure active from the depth of stability of the carpholite and persisting during subsequent exhumation the good preservation of carpholite and the absence of retrograde chloritoid below the shear zone show that exhumation occurred along a cooling path whereas the deeper units are exhumed along an isothermal path exhumation probably occurred during convergence and further nappe stacking during the earlier eocene these results suggest that pre collisional tectonic thinning of the penninic oceanic units may be more widespread and significant than generally recognized

seismic reflection evidence that the hayward fault extends into the lower crust of the san francisco bay area california this article presents deep seismic reflection data from an experiment across s an francisco peninsula in 1995 using large 125 to 500 kg explosive sources shot gathers show a mostly nonreflective upper crust in both the franciscan and salinian terranes juxtaposed across the san andreas fault an onset of weak lower crustal reflectivity beginning at about 6 sec two way travel time twtt and bright southwest dipping reflections between 11 and 13 sec twtt previous studies have shown that the moho in this area is no deeper than 25 km 8 to 9 sec twtt three dimensional reflection travel time modeling of the 11 to 13 sec events from the shot gathers indicates that the bright events may be explained by reflectors 15 to 20 km into the upper mantle northeast of the san andreas fault however upper mantle reflections from these depths were not observed on marine reflection profiles collected in san francisco bay nor were they reported from a refraction profile on san francisco peninsula the most consistent interpretation of these events from 2d raytracing and 3d travel time modeling is that they are out of plane reflections from a high angle dipping 70 to the southwest impedance contrast in the lower crust that corresponds with the surface trace of the hayward fault these results suggest that the hayward fault truncates the horizontal detachment fault suggested to be active beneath san francisco bay

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

new chronostratigraphy of the cretaceous paleogene boundary interval at bidart france abstract the cretaceous paleogene boundary section at bidart french basque coast was prevented for a long time to be considered as a reference section for the k t boundary due to a suspected hiatus comprising most of chron c29r in our new magneto and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small but consistent intensity excursion during laboratory demagnetization trends magnetite and maghemite are the main carriers of remanence with contribution of hematite to the magnetic mineralogy of the red brown lowermost danian and maastrichtian marly limestones magnetite is the main magnetic mineral in the white to pink danian limestones the nrm of marls and marly limestones is characterized by 1 a present day field overprint 2 a medium temperature 300 400 c reversed component interpreted as a relic of the characteristic remanent magnetization chrm carried by magnetite grains which are not completely oxidized into maghemite and 3 a high temperature up to 590 c normal component due to late diagenesis hematite growth the nrm of danian limestones is characterized by 1 a present day field overprint and 2 a chrm component of normal or reversed polarity the chrm directions allow a new correlation of the magnetic polarity sequence with the geomagnetic polarity time scale gpts chron c29r corresponds to the maastrichtian unit and extends up to 3 5 m above the k t boundary and chron c29n occurs at the top of the section there is no evidence for a hiatus at or below the k t boundary and the section does not extend up to chron c28r as previously suggested these new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper our study demonstrates that the section of bidart can be considered as a key section to study the k t boundary event s

new and revisited paleomagnetic data from permian triassic red beds two kinematic domains in the west central pyrenees abstract new paleomagnetic results of permian triassic red beds from bielsa aure and somport anayet sectors in the pyrenean axial zone are presented and combined with revisited and reviewed paleomagnetic data from the west central pyrenees the paleomagnetic data from the red beds vary between different sectors that share similar structural position i e whilst all sectors share the existence of a characteristic prefolding component a cenozoic postfolding component is found only in one area but not in other areas with similar structural position and kinematics previous paleomagnetic data to the west of the studied zone reveal a pre turonian remagnetization component but this secondary component is not found in an area that shares a similar structural position aure this variability suggests that the paleomagnetic behavior is the result of a series of factors related to the particular tectonic history of the region sedimentation burial and deformation during basin inversion and therefore paleomagnetic data demands careful assessment in order to unravel the kinematics of areas with equivalent structural positions in orogens however important kinematic implications that hold in the pyrenees can be inferred from the restoration of the permian triassic characteristic magnetizations to positions previous to the paleomagnetic rotations recorded by cretaceous or cenozoic paleomagnetic data the restoration reveals on one hand the lack of large rotations in the south pyrenenan zone except in the nogueras area according to bates 1989 and on the other hand the contrasting rotations recorded to the west of the studied zone in the paleozoic basque massifs pbm and the south pyrenean zone indicating the existence of two different tectonic domains in the west central sector of the pyrenees the strong clockwise rotations postdating the early cretaceous remagnetization that are recorded in the pbm and in the north pyreneean zone suggest a late dextral shear deformation affecting areas near the north pyrenean fault zone

relocation of teleseismically recorded earthquakes near tennant creek australia implications for midplate seismogenesis the three ms 6 tennant creek australia earthquakes of january 22 1988 were preceded by small and moderate earthquakes in 1986 and 1987 we have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986 1987 earthquakes the 1988 main shocks and aftershocks with respect to a system of surface fault scarps the 1986 1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps where fault segment boundaries at depth are implied by complexities in the distribution of scarps at the surface this suggests that the fault segments that ruptured in 1988 were already in existence in 1986 1987 which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults the redetermined 1986 1987 hypocenters are however also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust because they are situated on a regionally prominent gravity anomaly the teleseismically recorded seismicity of the tennant creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull the concentration of swarm earthquakes between two scarps is consistent with models in which precursory swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main shock faulting the time interval between swarm and main shocks is similar to intervals between intermediate term precursory swarms and main shocks in regions that have much higher rates of tectonic loading this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading but are determined primarily by time dependence of the failure process the spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks the set of teleseismically recorded aftershocks like the set of locally recorded aftershocks includes some events that occurred well away from the causative faults of the main shocks at the length scale of the 1988 main shock rupture the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

maastrichtian climate productivity and faunal turnovers in planktic foraminifera in south atlantic dsdp sites 525a and 21 abstract stratigraphic faunal and isotopic analyses of the maastrichtian at dsdp sites 525a and 21 in the south atlantic reveal a planktic foraminiferal fauna characterized by two major events an early late maastrichtian diversification and end maastrichtian mass extinction both events are accompanied by major changes in climate and productivity the diversification event which occurred in two steps between 70 5 and 69 1 ma increased species richness by a total of 43 and coincided with the onset of major cooling in surface and bottom waters and increased surface productivity the onset of the terminal decline in maastrichtian species richness began at 67 5 ma and the first significant decline in surface productivity occurred at 66 2 ma coincident maximum cooling to 13 c in surface waters and the reduction of the surface to deep temperature gradient to less than 5 c major climatic and moderate productivity changes mark the mass extinction and the last 500 kyr of the maastrichtian between 200 and 400 kyr before the k t boundary surface and deep waters warmed rapidly by 3 4 c and cooled again during the last 100 kyr of the maastrichtian surface productivity decreased only moderately across the k t boundary species richness began to decline during the late maastrichtian cooling and by k t boundary time the mass extinction had claimed 66 of the species viewed within the context of maastrichtian climate and productivity changes the k t mass extinction could have resulted from extreme environmental stress even without the addition of an extraterrestrial impact

does large igneous province volcanism always perturb the mercury cycle comparing the records of oceanic anoxic event 2 and the end cretaceous to other mesozoic events mercury hg is increasingly being used as a sedimentary tracer of large igneous province lip volcanism and supports hypotheses of a coincidence between the formation of several lips and episodes of mass extinction and major environmental perturbation however numerous important questions remain to be answered before hg can be claimed as an unequivocal fingerprint of lip volcanism as well as an understanding of why some sedimentary records document clear hg enrichment signals whilst others do not of particular importance is evaluating the impact of different volcanic styles on the global mercury cycle as well as the role played by depositional processes in recording global hg cycle perturbations here new mercury records of cretaceous oceanic anoxic event 2 oae 2 94 ma and the latest cretaceous 67 66 0 ma are presented oae 2 is associated with the emplacement of multiple predominantly submarine lips the latest cretaceous with subaerial volcanism of the deccan traps both of these connections are strongly supported by previously published trends towards unradiogenic osmium os isotope values in globally distributed sedimentary records hg data from both events show considerable variation between different locations attributed to the effectiveness of different sediment types in registering the hg signal with lithologically homogeneous records documenting more clear hg enrichments than sections with major changes in lithology such as limestones to claystones or organic rich shales crucially there is no geographically consistent signal of sedimentary hg enrichment in stratigraphic records of either oae 2 or the latest cretaceous that matches os isotope evidence for lip emplacement indicating that volcanism did not cause a global hg perturbation throughout the entire eruptive history of the lips formed at those times it is suggested that the discrepancy between os isotope and hg trends in records of oae 2 is caused by the limited dispersal range of hg emitted from submarine volcanoes compared to the global scale distribution of os a similar lack of correlation between these two proxies in uppermost cretaceous strata indicates that although subaerial volcanism can perturb the global hg cycle not all subaerial eruptions will do so these results highlight the variable impact of different volcanogenic processes on the efficiency of hg dispersal across the globe factors that could influence the impact of lip eruptions on the global mercury cycle include submarine versus subaerial volcanism volcanic intensity or explosivity and the potential contribution of thermogenic mercury from reactions between ascending magma and surrounding organic rich sediments

extinction survivorship and evolution of planktic foraminifera across the cretaceous tertiary boundary at el kef tunisia an expanded sediment record at el kef shows that the k t boundary extinctions of planktic foraminifera extend over an interval from 25 cm below the geochemical boundary ir anomaly to 7 cm above species extinctions appear sequential with complex large ornate forms disappearing first and smaller less ornate forms surviving longer the 14 species extinctions below the boundary appear unrelated to an impact event cretaceous species survivorship is greater than previously assumed about 10 species survive 22 into subzone p1a globigerina eugubina all cretaceous survivors are small primitive forms which are generally smaller than their ancestors in cretaceous sediments species evolution after the k t event occurs in two pulses the first new paleocene species evolve in the basal black clay zone po immediately after the major cretaceous extinctions evolving species are small and primitive similar to cretaceous survivors the second pulse in species evolution occurs in the lower part of subzone p1b with the appearance of larger more diverse species the first major increase in carbonate sedimentation and productivity occurs at this time and signals the recoveyr of the ecosystem nearly 300 000 years after the k t event the species extinctions prior to the generally assumed impact event implied by the ir anomaly and the long recovery period of the ecosystem thereafter cannot be explained by a single impact but suggest that multiple causes may be responsible such as climatic changes volcanism a sea level drop production of warm saline bottom water and the chemical consequences associated with increased salinity

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

crustal structure beneath the strait of juan de fuca and southern vancouver island from seismic and gravity analyses 1 wide angle and vertical incidence seismic data from seismic hazards investigations in puget sound ships gravity modeling and seismicity are used to derive two dimensional crustal models beneath the strait of juan de fuca the eocene volcanic crescent siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater for the northern strait a weak midcrustal reflector dipping east from 12 to 22 km depth is inferred from wide angle reflections a stronger deeper reflector dipping eastward from 23 to 36 km depth is associated with the top of reflector band e a zone of high reflectivity on coincident multichannel seismic mcs data interpreted as a shear zone a high velocity zone 7 60 0 2 km s 1 between these reflectors is interpreted as a localized slice of mantle accreted with the overlying crescent siletz terrane for the southern strait no deep high velocity layer is observed and the e band reflectivity is weaker than to the north a strong deep reflector interpreted as the oceanic moho dips eastward from 35 to 42 km seismicity within the subducting slab occurs mainly above the inferred oceanic moho gravity modeling constrained by the wide angle seismic models and seismicity is consistent with the inferred large thickness of crescent siletz and high density rocks 3030 kg m 3 in the lower crust

integrated paleocene calcareous plankton magnetobiochronology and stable isotope stratigraphy dsdp site 384 nw atlantic ocean abstract at deep sea drilling site 384 j anomaly ridge grand banks continental rise nw atlantic ocean paleocene nannofossil chalks and oozes 70 m thick are unconformably disconformably underlain 168 m upper maastrichtian and overlain 98 7 m upper lower eocene by sediments of comparable lithologies the chalks are more indurated in stratigraphically higher levels of the paleocene reflecting increasing amounts of biosiliceous radiolarians and diatoms components this site serves as an excellent location for an integrated calcareous and siliceous microfossil zonal stratigraphy and stable isotope stratigraphy we report the results of a magnetostratigraphic study which when incorporated with published magnetostratigraphic results reveals an essentially complete magnetostratigraphic record spanning the interval from magnetochron c31n late maastrichtian to c25n partim late paleocene thanetian integrated magnetobiochronology and stable isotope stratigraphy support the interpretation of and constrain the estimated duration of a short hiatus 0 9 my within the younger part of chron c29r including the k p boundary and an 6 my hiatus separating upper paleocene magnetozone c25n and upper lower eocene magnetozone c22r sediments some 30 planktonic foraminiferal datum levels including the criteria used to denote the paleocene planktonic foraminiferal sub tropical zonal scheme of berggren and miller micropaleontology 34 4 1988 362 380 and berggren et al sepm spec publ 54 1995 129 212 geol soc am bull 107 11 1995 1272 1287 and nearly two dozen calcareous nannoplankton datum levels have been recognized and calibrated to the magnetochronology planktonic foraminiferal subzones p4a and p4b of upper paleocene zone p4 are emended redefined based on the discovery of a longer stratigraphic extension of acarinina subsphaerica into at last magnetozone c25n stable isotope stratigraphies from benthic foraminifera and fine fraction

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

magnetostratigraphy and rock magnetism of the ilerdian stratotype at tremp spain abstract the first magnetic polarity stratigraphy of the ilerdian stratotype locality the tremp section spain has been established in total 315 samples from 66 levels have been demagnetized both thermally and by alternating field various types of rock magnetic behaviors and low and high temperature magnetization components have been distinguished and have been correlated with the three sampled rock types clays marls and fine grained sandstones and coarse grained sandstones the paleomagnetic directions have been classified according to their reliability in representing a primary magnetization the magnetostratigraphy of the tremp section is characterized by a long reversed polarity zone interrupted by a normal zone we conclude that the normal polarity interval found in the ilerdian stratotype may represent chron 24 2 the lower and upper reversed intervals of the section are then respectively chrons 24 1r and 24r