Magnetic Susceptibility Logs Research Articles

The Upper Ordovician Pagoda positive δ13C excursion (PPCE) on the Upper Yangtze Platform, South China

The continuous and well exposed sections of the Upper Ordovician on the Upper Yangtze Platform were surveyed and sampled for δ13C and δ18O isotope and conodont fossils. The geochem-stratigraphic work in the Pagoda Formation has resulted in the recognition of a positive δ13C excursion, the Pagoda Positive δ13C Excursion (PPCE). The PPCE is a distinctive positive excursion occurring in the Middle-Late Katian, consists of three minor excursions ranging from 1.5 ‰ to 3 ‰, which has been identified in all the study sections on the Upper Yangtze Platform. The PPCE interval ranges through the conodont Hamarodus brevirameus and Protopandeordus inscrulptus conodont zones in ascending order, which indicates that the PPCE develops in the Middle-Late Katian of the Late Ordovician, while the Guttenberg δ13C Excursion (GICE) in the Baltoscandia and America extended from the Late Sandbian to the Early Katian. Spectral analysis of high-resolution magnetic susceptibility (MS) logs for the four study sections shows consistent wavelengths of ca. 2.0, 0.5, 0.18 and 0.15 m. The outcrop depth-series were tuned using 405-kyr long-eccentricity cycles and the astronomical tuning yields the duration of 3.7 to 4.4 myr for the PPCE event in different sections. This is the first documentation of the PPCE event on the Upper Yangtze Platform according to the geochemo-bio-cyclostratigraphy evidences, being identified within all portions of this platform in South China. It revised the previous description of the positive δ13C excursion of the Pagoda Formation on the Upper Yangtze Platform. The trigger of the PPCE was also briefly discussed in this paper.

Late Quaternary activity of the Xiadian west concealed fault east to Beijing and its tectonic significance.

Beijing plain is a strong earthquake tectonic area in China. There was a Sanhe-Pinggu earthquake with Ms8 that happened in1679. The seismogenic fault of this earthquake is called Xiadian fault. Our work found fault with a similar strike and opposite dip in the west of the Xiadian fault, which is called the Xiadian west fault in this paper. Six shallow seismic profiles have been constructed to determine the location of the fault in Sanhe city, and the late Quaternary activity of the fault is studied with the method of combined drilling, magnetic susceptibility logging, and luminescence dating. The results of shallow seismic exploration show that the fault is zigzag and generally strikes NE and inclines NW. According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12 m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is 36.52 ±5.39 ka. The vertical slip rate has been about 0.075± 0.023 mm/a since the late Pleistocene and about 0.058 ± 0.030 mm/a since the late period of the late Pleistocene. It can be inferred that the Xiadian west fault is probably a part of the seismogenic structure of the Sanhe-Pinggu Ms8 earthquake that happened in 1679. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the Xiadian west fault.

An astronomical age-depth model and reconstruction of moisture availability in the sediments of Lake Chalco, central Mexico, using borehole logging data

Understanding the moisture history of low latitudes from the most recent glacial period of the latest Pleistocene to post-glacial warmth in continental tropical regions is hampered by the lack of continuous time series. We conducted downhole spectral gamma (γ) ray and magnetic susceptibility logs over 300 m of lacustrine deposits of Lake Chalco (Mexico City) to reconstruct an age-depth model using an astronomical and correlative approach, and to reconstruct long-term moisture availability. Our results suggest that the Lake Chalco sediments contain several rhythmic alternations with a quasi-cyclic pattern comparable to the Pleistocene benthic stack. This allows us to calculate a time span of about 500,000 years for this sediment deposition. We developed proxies for moisture, detrital input, and salinity, all based on the physical properties of γ-ray spectroscopy and magnetic susceptibility. Our results indicate that Lake Chalco formed during Marine Isotope Stage 13 (MIS13) and the lake level gradually increased over time until the interglacial MIS9. Moisture content is generally higher during interglacials than during glacials. However, two periods, namely MIS6 and MIS4, have higher moisture contents. We developed a model by comparing the obtained moisture proxy with climatic drivers, to understand how different climate systems drove effective moisture availability in the Chalco sub-basin over the past 500,000 years. Carbon dioxide, eccentricity, and precession are all key drivers of the moisture content of Lake Chalco over the past 500,000 years.

Interval logging in blast holes for 3D geological modeling of blasting blocks at Lebedinsky GOK

The current practice of geophysical logging in blast holes at Lebedinsky GOK reduces to evaluation of an average content of magnetic iron per test bench. In the meanwhile, the measurement procedure in use (magnetic susceptibility logging) allows obtaining more comprehensive data from each blast hole as the measurement frequency is every 10 cm. Interpretation of geological structure of blasting blocks uses 2D models (or a charts). These methods are incapable to detect in full ore and barren rock interfaces, to locate ‘blind’ diorite dykes and to pattern qualities within process grades. The authors have developed a batch processing procedure for well logging data in Surpac environment. In this case, 3D interpretation of geological structure of an extraction block is implemented using the interval logging data on magnetic susceptibility in blast holes. The implemented analysis shows that geophysical logging fails to identify some thin dykes, or identifies them unreliably. The current logging practice, when geological interpretation has only one averaged value per hole, is incapable to ensure sufficient accuracy of interpretation of geological boundaries inside blasting blocks. For more particular conclusions, the research procedure needs further study and refinement. The use of the proposed procedure for geophysical logging data can accelerate processing of geophysical test data from blast holes, improve accuracy of interpretation of geological boundaries, correlate the data with specification of geological structure of blocks and reduce loss and dilution in the long run.

Developing and expanding the Late-Glacial and Holocene tephrochronological framework of France: A new contribution from the Chaîne des Puys volcanic field in the Massif Central

The aim of this paper is to highlight new occurrences of tephra and cryptotephra layers from three peatbogs located southeastern from the French Massif Central in France. These tephras were evidenced using magnetic susceptibility logging, scanner imagery and chemical analyses by X-Ray Fluorescence, and characterized by their glass chemistry. Originated from the Chaîne des Puys volcanic field, these six new tephras allow for the definition of a more detailed regional tephrostratigraphic framework. Most of these tephras were deposited between ca. 13,500 and 9500 yrs cal. BP, and the last one was deposited between 6400 and 5920 yrs cal. BP. Some tephras of this framework were previously described in the Jura Mountains, the Northern Alps and the Western Swiss Plateau, and allow its correlation to the Northern European context established by previous studies.

Post-remediation geophysical assessment: Investigating long-term electrical geophysical signatures resulting from bioremediation at a chlorinated solvent contaminated site

There is a growing need to assess long-term impacts of active remediation strategies on treated aquifers. A variety of biogeochemical alterations can result from interactions of the amendment with the aquifer, conceivably leading to a geophysical signal associated with the long-term alteration of an aquifer. This concept of post-remediation geophysical assessment was investigated in a shallow, chlorinated solvent-contaminated aquifer six to eight years after amendment delivery. Surface resistivity imaging and cross-borehole resistivity and induced polarization (IP) imaging were performed on a transect that spanned treated and untreated zones of the aquifer. Established relationships between IP parameters and surface electrical conductivity were used to predict vertical profiles of electrolytic conductivity and surface conductivity from the inverted cross-borehole images. Aqueous geochemistry data, along with natural gamma and magnetic susceptibility logs, were used to constrain the interpretation. The electrical conductivity structure determined from surface and borehole imaging was foremost controlled by the electrolytic conductivity of the interconnected pore space, being linearly related to fluid specific conductance. The electrolytic conductivity (and thus the conductivity images alone) did not discriminate between treated and untreated zones of the aquifer. In contrast, inverted phase angles and surface conductivities did discriminate between treated and untreated zones of the aquifer, with the treated zone being up to an order of magnitude more polarizable in places. Supporting aqueous chemistry and borehole logging datasets indicate that this geophysical signal from the long-term impact of the remediation on the aquifer is most likely associated with the formation of polarizable, dispersed iron sulfide minerals.

Cyclostratigraphy and paleoenvironmental inference from downhole logging of sediments in tropical Lake Towuti, Indonesia

Lake Towuti is located on central Sulawesi/Indonesia, within the Indo Pacific Warm Pool, a globally important region for atmospheric heat and moisture budgets. In 2015 the Towuti Drilling Project recovered more than 1000 m of drill core from the lake, along with downhole geophysical logging data from two drilling sites. The cores constitute the longest continuous lacustrine sediment succession from the Indo Pacific Warm Pool. We combined lithological descriptions with borehole logging data and used multivariate statistics to better understand the cyclic sequence, paleoenvironments, and geochronology of these sediments. Accurate chronologies are crucial to analyze and interpret paleoclimate records. Astronomical tuning can help build age-depth models and fill gaps between age control points. Cyclostratigraphic investigations were conducted on a downhole magnetic susceptibility log from the lacustrine facies (10–98 m below lake floor) from a continuous record of sediments in Lake Towuti. This study provides insights into the sedimentary history of the basin between radiometric ages derived from dating a tephra layer (~ 797 ka) and C14-ages (~ 45 ka) in the cores. We derived an age model that spans from late marine isotope stage (MIS) 23 to late MIS 6 (903 ± 11 to 131 ± 67 ka). Although uncertainties caused by the relatively short record and the small differences in the physical properties of sediments limited the efficacy of our approach, we suggest that eccentricity cycles and/or global glacial-interglacial climate variability were the main drivers of local variations in hydroclimate in central Indonesia. We generated the first nearly complete age-depth model for the lacustrine facies of Lake Towuti and examined the potential of geophysical downhole logging for time estimation and lithological description. Future lake drilling projects will benefit from this approach, since logging data are available just after the drilling campaign, whereas core descriptions, though more resolved, only become available months to years later.

What over 100 drillings tell us: a new method for determining the Koenigsberger ratio of soils from magnetic mapping and susceptibility logging

AbstractWe investigate the relative fractions of remanent and induced magnetization of the fillings of neolithic long pits in order to develop remanent magnetization as an additional parameter for the archaeological interpretation of magnetic maps. We determine the Koenigsberger ratio – the ratio between induced and remanent magnetization intensities – for key targets by combining magnetic mapping with downhole measurements of susceptibility, numerical modelling, and inversion computations. The susceptibility data were acquired in drill holes along profiles crossing the targets identified by magnetic mapping. The targets of this exemplary study are house‐accompanying pits at the Linearbandkeramik site Vráble ‘Farské’. For this purpose, we conducted auger drillings with a point distance of 25 cm and measured the susceptibility with a downhole susceptometer. The resulting two‐dimensional susceptibility distributions were used to calculate synthetic magnetic anomalies corresponding to the case of solely induced magnetization. The comparison to the observed magnetic data showed a considerable discrepancy that can only be explained with remanent magnetization. To determine the Koenigsberger ratio we developed a new interpretation approach, using parts of the measured susceptibility distribution as a basis function. The free parameters of this numerical problem are determined by non‐linear inversion. We applied the novel approach to six exemplary profiles and found Koenigsberger ratios between 1.6 and 10.5 with the majority of the values being smaller than 4. These values apply to soil volumes with susceptibility values larger than 27–160 × 10‐5 SI. Laboratory measurements on soil samples were used to examine the possible causes of the observed magnetization. The analyses suggest that the increase in susceptibility and remanent magnetization in the pits is caused by an increase of the population of magnetotactic bacteria and deposition of magnetized material, followed by the alignment of the ferrimagnetic iron compounds in the waterlogged environment of the pits and accumulation of viscous remanent magnetization.

A multi-proxy magnetic approach for monitoring large-scale airborne pollution impact

The interpretive utility of environmental magnetic proxies for investigating airborne particulate matter (PM) pollution impact is restricted by differences in soil composition, land cover and land use. For soil magnetic applications, land use strongly influences magnetic particle distribution down the soil profile, even in homogeneous soil environments. Here, an adaptive approach is engineered to provide accurate magnetic proxy information for pollution monitoring across different land use types. In an 81-km2 area between two industrial harbours, the irregular distribution of forests, arable lands, pasture and residential areas prevented robustly relating topsoil magnetic susceptibility data to known pollution impacts. Although normalized topsoil susceptibility values showed improved potential for deriving airborne pollution impacts, optimal results were obtained by depth-integrating magnetic susceptibility logs, revealing long-term impacts of both active and decommissioned industrial facilities. Complementing soil magnetic observations, active and passive (bio)magnetic monitoring allowed discriminating short-term pollution patterns and evaluating changes in PM impact across the study area. Hereby, active PM receptors (strawberry leaves and plastic coated cardboards (PCCs)) provided promising results, yet passive receptors allowed estimating pollution impacts more efficiently. For the latter, species-independent grass leaf sampling reflected airborne PM depositional patterns most accurately, whereas wiped anthropogenic surfaces proved too sensitive to wash-off.

Evaluation of disturbance induced on soft offshore sediments by two types of gravity piston coring techniques

Sample disturbance is still a key issue in offshore investigations, especially when logistic and financial limitations do not allow the use of drilling equipment. This paper focuses on the comparison between the disturbance induced by a conventional free-fall piston corer (FF) and a modified piston corer (AD) equipped with a velocity control (Angel Descent method). Twin core samples were retrieved in two successions of pelitic sediments with a prevailing non-clayey fraction and a non-negligible sandy fraction. Comparison was based on different acquisition, physical and mechanical parameters ranging from accelerometer data to magnetic susceptibility logs and geotechnical parameters from laboratory investigations, including oedometer compression tests and cyclic simple shear tests. Accelerometer data highlighted the sharp reduction in velocity obtained for AD samples. Magnetic susceptibility logs, characterized by a pattern of peaks induced by several volcaniclastic levels present in the succession, indicated that the AD method significantly reduces core shortening. Among geotechnical investigations, cyclic shear tests provided small-strain shear moduli always higher in AD samples, whilst the response of oedometer compression tests was equivocal. In fact, methods for assessing sample disturbance have demonstrated to bear limited effectiveness when applied to soils with relatively low clay content and significant overconsolidation as it is the case of the studied sediments.

AGE AND MODE OF THE MIDDLE MIOCENE MARINE FLOODING OF THE PANNONIAN BASIN—CONSTRAINTS FROM CENTRAL SERBIA

The timing and mode of the marine flooding of the southern margin of the Pannonian basin in SE Europe is still a matter of debate. In central Serbia, integrated bio-magnetostratigraphic data and quantified high-resolution records are completely missing. Here, we provide paleoenvironmental and paleoecological constraints from the Slanci section located near Belgrade that has an excellent preservation of micro-fauna and flora, i.e., planktonic and benthic foraminifera and calcareous nannoplankton. We integrate their quantified records with sedimentological, natural gamma radioactivity and magnetic susceptibility logs and include the non-quantified records of mollusks, corals, and ostracods to reconstruct the regional depositional history. The section shows upper bathyal to outer shelf depositional settings and alternating nutrient bottom conditions. The shallowing upwards trend marked by increasing terrestrial input, attributes it to the early Highstand Systems Tract of the first marine third order sequence in this part of the Pannonian Basin. We infer that the marine flooding in central Serbia took place at an age of ∼ 14 Ma, slightly predating the Langhian/Serravallian boundary (13.82 Ma) and the Badenian Salinity Crisis (∼ 13.8–13.4 Ma) in the Central Paratethys. Our results support an eastwards directed, tectonically forced, flooding of the southern Pannonian Basin. This major paleoenvironmental turnover was forced by syn-rift tectonics in the Pannonian Basin system, which started in the late early Miocene in the Styrian Basin (SE Austria) and ended more than 3 Myr later in the late middle Miocene in the Morava depression (E Serbia).

Templates for Quantifying Clay Type and Clay Content from Magnetic Susceptibility and Standard Borehole Geophysical Measurements

This paper proposes a rapid means of identifying clay type and quantifying clay content from new template crossplots that compare magnetic susceptibility measurements with standard borehole well log data. The templates are similar in format to standard industry charts, but have a number of advantages over the commonly used charts. Laboratory measurements of magnetic susceptibility on core samples and drill cuttings have recently shown strong correlations with key petrophysical parameters, particularly clay content and fluid permeability [1] [2]. A new template crossplot between magnetic susceptibility and borehole spectral gamma ray log data can firstly help to quickly identify the types of clay present in the formation. Additional new template crossplots between magnetic susceptibility and borehole bulk density data allow the mineral contents and porosities of binary mixtures of clay minerals and matrix minerals (such as illite clay + quartz) to be rapidly quantified. The templates can use ambient (room temperature) magnetic susceptibility data from measurements on core samples or drill cuttings in the laboratory or at the wellsite. Furthermore, the paper shows how the templates can potentially be extended to utilize borehole magnetic susceptibility data for in situ estimations of the type and content of clay. This requires accounting for the temperature dependence of the magnetic susceptibility of paramagnetic minerals (such as illite clay), which varies with depth in a borehole. Whilst borehole magnetic susceptibility measurements are rarely part of standard well logging operations, they could be a potentially useful tool for in situ clay type and content quantification, which in turn can help predict fluid permeability.

Geophysical investigation to support characterisation of structurally controlled groundwater flow into an open pit mine

Efficient dewatering operations depend on reliable predictions of expected inflow and likely water level behaviour. These predictions stem from the conceptual understanding of the hydrogeology of the area, which itself is derived from studies into the aquifer extents, hydraulic parameterisation and connectivity with adjacent aquifer units. Due to their isolated nature, structural controls on these inputs to the conceptual understanding are amongst the most challenging to determine. An investigation was undertaken on structurally controlled flow into an open pit iron ore mine in the Pilbara region of Western Australia. Bore yields, groundwater salinity and water level behaviour of in-pit bores unmistakably indicated flows in excess of normal aquifer throughflow. Airborne magnetic data allowed regional identification of a potential lineament which was confirmed with high density grade control drilling results. These indicated the presence of a trough-like mineralised feature likely to enhance connection to a regional aquifer system down dip of the iron ore body, or with a fractured rock aquifer beneath the ore body. A detailed ground magnetic survey was conducted along strike from the interpreted lineament, drastically improving on the aeromagnetic data, further validating the conceptualisation of the structure and providing greater accuracy with which to target future hydrogeological works around the current pit. Subsequent quantitative magnetic modelling constrained with close spaced drilling information and downhole magnetic susceptibility logging, increased the understanding of the characteristic basement magnetic response, and encourages the use of the magnetic method for local hydrogeological studies.

Prediction of sonic velocities from other borehole data: An example from the Kevitsa mine site, northern Finland

ABSTRACTP‐wave and S‐wave velocities are vital parameters for the processing of seismic data and may be useful for geotechnical studies used in mine planning if such data were collected more often. Seismic velocity data from boreholes increase the robustness and accuracy of the images obtained by relatively costly seismic surface reflection surveys. However, sonic logs are rarely acquired in boreholes in‐and‐near base metal and precious metal mineral deposits until a seismic survey is planned, and only a few new holes are typically logged because the many hundreds of holes previously drilled are no longer accessible. If there are any pre‐existing petrophysical log data, then the data are likely to consist of density, magnetic susceptibility, resistivity and natural gamma logs. Thus, it would be of great benefit to be able to predict the velocities from other data that is more readily available.In this work, we utilize fuzzy c‐means clustering to build a “fuzzy” relationship between sonic velocities and other petrophysical borehole data to predict P‐wave and S‐wave velocity. If boreholes with sonic data intersect most of the important geological units in the area of interest, then the cluster model developed may be applied to other boreholes that do not have sonic data, but do have other petrophysical data to be used for predicting the sonic logs. These predicted sonic logs may then be used to create a three‐dimensional volume of velocity with greater detail than would otherwise be created by the interpolation of measured sonic data from sparsely located holes.Our methodology was tested on a dataset from the Kevitsa Ni‐Cu‐PGE deposit in northern Finland. The dataset includes five boreholes with wireline logs of P‐wave velocity, S‐wave velocity, density, natural gamma, magnetic susceptibility and resistivity that were used for cluster analysis. The best combination of input data for the training section was chosen by trial and error, but differences in the misfit between the various training datasets were not particularly significant. Our results show that the fuzzy c‐means method can predict sonic velocities from other borehole data very well, and the fuzzy c‐means method works better than using multiple linear‐regression fitting. The predicted P‐wave velocity data are of sufficient quality to robustly add low‐frequency information for seismic impedance inversion and should provide better velocity models for accurate depth conversion of seismic reflection data.

The Sarmatian/Pannonian boundary at the western margin of the Vienna Basin (City of Vienna, Austria)

Abstract Sarmatian and Pannonian cores, drilled at the western margin of the Vienna Basin in the City of Vienna, reveal a complex succession of marine and lacustrine depositional environments during the middle to late Miocene transition. Two Sarmatian and two Pannonian transgressive-regressive sequences were studied in detail. Identical successions of benthic faunal assemblages and similar patterns in magnetic susceptibility logs characterise these sequences. This allows a correlation of the boreholes over a distance of ~3.5 km across one of the major marginal faults of the Vienna Basin. Biostratigraphic data, combined with rough estimates of sedimentation rates, reveal large gaps between these sequences, suggesting that only major transgressions reached this marginal area. In particular, during the Sarmatian-Pannonian transition, the basin margin completely emerged and turned into a terrestrial setting for at least 600 ka.

87Sr/86Sr dating and preliminary interpretation of magnetic susceptibility logs of giant piston cores from the Rio Grande Rise in the South Atlantic

Giant piston cores recovered from shallow depths (<1200 mbsl) on the northern flank of the Rio Grande Rise, bathed today in intermediate waters (AAIW, UCDW), have uncovered new stratigraphic and paleoceanographic aspects of the Plio-Pleistocene in the South Atlantic. Based on strontium isotope analysis of well-preserved foraminifera-rich sediment a stratigraphy was developed from lowess curve fitting of the data and an optimized matching with an internationally recognized timescale of 87Sr/86Sr seawater variation through geological times. Depth-to-age conversion of the magnetic susceptibility logs was implemented based on the identification of correlative peaks between cores and the developed 87Sr/86Sr age model. The influence of Northern Hemisphere glaciation is reflected in these new stratigraphic logs by a gradual increase from ∼2.7 Ma in the lower signal of magnetic susceptibility (below background level), to values approaching the arithmetic means, likely reflecting an overall increase in terrigenous input. The Rio Grande Rise cores have very low Plio-Pleistocene sedimentation rates (∼0.4–0.8 cm/ka), similar to gravity cores from the oligotrophic subtropical South Atlantic (below ∼2000 mbsl), and for which an inverse correlation between carbonate content and magnetic susceptibility was established. The coring depths on the Rio Grande Rise encompass strong gradients in oxygen concentration and other seawater parameters that define today's AAIW/UCDW transition. Depth-dependent variation in sedimentation rates since the onset of Northern Hemisphere glaciation coincides with the incursion of intermediate waters (UCDW, AAIW) in response to the overall reduction of NADW export to the Southern Ocean. Background levels of magnetic susceptibility in the cores suggest that this variation is mainly attributed to terrigenous input. The source region of this material has yet to be traced by considering in particular the mineral composition and paramagnetic properties of the detrital clays.

Petrophysical record of the Late Silurian shallow-water carbonate facies across the Lau Event (Prague Synform, Czech Republic) and dynamic time warping alignment of the magnetic susceptibility logs

Abstract High-resolution petrophysical logging magnetic susceptibility measurements (MS) and gamma-ray spectrometry (GRS), supplemented by rock magnetic measurements (anhysteretic and isothermal remanent magnetizations (ARM/IRM), frequency-dependent MS (FDMS)) and MS logs alignment using the dynamic time warping (DTW) algorithm, were constrained by upgraded conodont biozonation across the Lau Event interval (LEI). Two sections (Pozary and Muslovka) representing shallow-water facies in the Prague Synform were studied. Conodont data and DTW alignment show that Muslovka represents a less complete succession when compared to the Pozary. DTW alignment of the MS logs shows perfect match of the logs and detected six prominent and several minor gaps. MS logs show elevated values and triple peaks at both sections across the LEI which start in the uppermost part of the siluricus Biozone. Magnetite was identified as the main carrier of the MS signal at both sections using ARM and IRM. FDMS revealed occurrence of superparamagnetic (SP) particles at both sections in all selected samples. Furthermore, a higher contribution of SP particles to the MS signal is recorded below the Lau Event interval in the Pozary section. GRS logs show depletion in U concentrations across the LEI at both sections.

Astronomically forced cyclicity in the Upper Ordovician and U–Pb ages of interlayered tephra, Oslo Region, Norway

The Late Ordovician world experienced a series of huge volcanic eruptions, recorded as the big Deicke, Millbrig and Kinnekulle bentonites, together with numerous thinner beds. The Kinnekulle event can be traced across northwestern Europe. U–Pb zircon ages are here presented of both the Kinnekulle K-bentonite and the uppermost recorded tephra layer in the Upper Ordovician of the Oslo Region, Norway. The tephras are located in the upper part of the Arnestad Formation (Sandbian) south of Oslo and gave ages of 454.52±0.50Ma (the Kinnekulle K-bentonite) and 453.91±0.37Ma (the upper Grimstorp K-bentonite). The dated tephras are separated by a 7m thick shale succession with subordinate nodular limestone beds. Although the two U–Pb ages overlap within errors, statistical considerations indicate a most likely time interval of about 600kyr between the tephras. High-resolution magnetic susceptibility logging in the same section shows cycles that likely represent changes in sediment supply in response to astronomical forcing. Spectral analysis shows the presence of long (400kyr) and short (100kyr) eccentricity bands, and obliquity components in the 30kyr band. Precessional cycles are not detected. Based on this method, it is possible to estimate a time interval of 766kyr between the two tephra events, and the radiometric dating therefore does not exclude a Milankovitch interpretation of the cycles. This opens new possibilities for understanding the evolution of one of the world's best preserved Ordovician marine systems.

Magnetic susceptibility logging of Chicxulub proximal impact breccias in the Santa Elena borehole: implications for emplacement mode

Magnetic susceptibility logging is used to study the impact breccias in the Chicxulub crater. The basic premise is that the high contrasts in magnetic properties can be used to characterize the breccias. The Santa Elena borehole was drilled 110 km radial distance from crater center and sampled a 172 m thick sequence of impact breccias, between 332 and 504 m depth. Breccia units are distinguished from differences in composition, size, and relative contents of clasts, type of matrix and textural and lithological assemblages, which can be resolved in the susceptibility logs. The whole-core log shows characteristic variation patterns with high, intermediate and low susceptibilities. High resolution logging of matrix and clasts records the heterogeneous nature of impactites, with higher variability at smaller spatial scales. Measurements confirm that diamagnetic susceptibilities characterize the carbonate clasts, high susceptibilities the basement granitic clasts and intermediate values the silicate melt-rich and silicate-poor matrix. Intermediate variable susceptibilities characterize breccias rich in melt particles. Correlation of matrix and clast logs with whole-core log shows that signal is controlled by the matrix. Logs for clast shows a discrete distribution with peaks of intermediate to high values, which correlate with large clast distributions. The ejecta blanket includes the fallback suevites rich in silicate melt particles and shocked minerals, the high temperature vapor deposits from ejecta curtain collapse and high velocity basal flows, and the carbonate rich deposits from lateral basal flows and secondary cratering. Late fallback suevites record minor turbulent conditions resulting from progressive cooling of the ejecta plume.

The Wood Lake, Ontario, H4 ordinary chondrite, a new Canadian meteorite

The Wood Lake meteorite was found in the Muskoka region of Ontario in July 2003. We examine the mineralogy, physical properties, shock, and weathering state of this recently reported Canadian meteorite. Bulk physical properties for Wood Lake (bulk density 3.57 ± 0.10 g/cm3, grain density 3.64 ± 0.01 g/cm3, and magnetic susceptibility log χ = 5.17) are typical of slightly weathered H chondrite falls and consistent with visual observations of its observed low weathering state (W1). Reconnaissance X-ray micro computed tomography of a Wood Lake fragment yields a similar bulk density (3.56 g/cm3) and allows for a three-dimensional description of the meteorite. Petrographic and electron probe microanalysis examination of the Wood Lake meteorite reveal sharp definition of chondrule margins, occasional mesostasis chondrule glass, only minor development of recrystallized matrix feldspar, and little variation in silicate chemical compositions, indicating Wood Lake to be an H4 ordinary chondrite. Mineral chemistry for Wood Lake olivine (Fa18.6 ± 0.7, n = 26) and low-Ca pyroxene (Fs17.5 ± 2.6, n = 16) agrees well with H chondrite averages for these minerals. The bulk of the stone has undergone a low to moderate level of shock (S3), as indicated in thin section by undulose optical extinction and the presence of planar dislocations in olivine and pyroxene grains. Moderate distortion of olivine crystal structures observed using in situ micro X-ray diffraction quantifies the shock deformation as a full width half maximum (FWHMχ) of 3.38° ± 1.13° (16 reflections from seven olivine grains), consistent with shock stage S3 ordinary chondrites. Wood Lake is confirmed as a new Canadian meteorite find that likely arrived on Earth not long before it was recovered.