Rock Magnetic Evidence Research Articles

Rock magnetic evidence of tectonic control on the sedimentation and diagenesis in the Andaman Sea over ~1 million years

We evaluate the sedimentary (magnetic, mineralogical, geochemical, grain size) records of a long sediment core (NGHP-01-17 A) to elucidate the tectonic control on the sedimentation and diagenesis in the Andaman Sea (AS) over the past ~ 1 Ma. Two distinct sedimentary magnetic zones (Z-I and Z-II) are identified based on the rock magnetic signatures. A significant shift in the depositional environment is observed around ~90 ka which is marked by a rapid change in magnetite susceptibility (χlf), Fe, Ti and organic carbon content in Z-I. A trend of upward increase in χlf from ~90 ka to 3 ka in core NGHP-01-17 A suggest increase in detrital flux to the AS and represents the onset of a new sedimentary regime following most recent event of tectonic uplift in Indo-Burman Ranges (IBR) which occurred at ~ 150 ka. Methane influenced progressive diagenetic dissolution of detrital iron oxides and subsequent precipitation of iron sulfides in Z-II is clearly evident in the rock magnetic and mineralogical data. Multiple bands of diagenetically formed iron sulfide nodules at different periods in Z-II indicate episodic intensification of anaerobic oxidation of methane (AOM) due to enhanced methane-flux. We propose that fluid migration forced by tectonic processes facilitated episodic release of methane from deep reservoir that led to the formation of iron-sulfide nodules in the AS.

Rock magnetic evidence for a middle Holocene transition in marine sediments from La Paz basin, southern Gulf of California

Results of a rock magnetic study of a marine sediment core from La Paz Basin are used to investigate the Holocene paleoclimate in the Gulf of California. Chronological control based on AMS radiocarbon dating shows that the sequence covers the past ~7650 cal yr BP. The coercivity, magnetization intensity, susceptibility and magnetic hysteresis loops point to dominant fine-grained magnetite and titanomagnetites as main magnetic carriers. In the magnetic hysteresis ratio plot, magnetization and coercivity ratios fall in the pseudo-single domain field. High magnetic susceptibilities and magnetization intensities characterize the upper core section, with lower values downcore on a low amplitude oscillation trend. The late Holocene is marked by low susceptibilities, frequency-dependent, paramagnetic slope factors and variable hysteresis parameters with low Mr/Ms magnetization ratios, low Bc and Bcr coercivities and high Bcr/Bc ratios. The lower core section in the ~7650-5000 cal yr BP interval shows increased susceptibilities as compared to the upper sections with lower values. The magnetic hysteresis logs for the interval around 5000-4000 cal yr BP show some samples with distinct lower magnetization (Mr/Ms) and higher coercivity (Bcr/Bc) ratios, which characterize the interval from the downcore variation trend. The pattern in the magnetic properties is interpreted in terms of a change in precipitation from high to low humidity from the middle to the late Holocene. The sediment section records a middle Holocene climate transition in the southern Gulf ~5000-4000 cal yr BP, supporting a northward migration of climate patterns along the western North America margin, from the southern Gulf of California to Alaska.

Paleo-earthquakes revealed by rock magnetic evidence from the Anxian-Guanxian Fault, Sichuan Province, China

To determine the rock magnetic characteristics of this co-seismic rupture generated by the 2008 Wenchuan earthquake in Sichuan Province, China, we excavated a trench on the Anxian-Guanxian fault. High-resolution magnetic susceptibility and other rock magnetic measurements were conducted on the field section and on samples from the trench. We divided the fault rocks into 8 units according to lithology and measurements of magnetic susceptibility were made on fault rocks within different units, both in the field and in the laboratory. The results demonstrate that the highest magnetic susceptibility occurs in the red gouge (64.08 × 10−6 SI for surface magnetic susceptibility and 30.61 × 10−8 m3/kg for mass magnetic susceptibility), while relatively high magnetic susceptibility occurs in the black gouge (14.31× 10−6 SI for surface magnetic susceptibility and 7.15 × 10−8 m3/kg for mass magnetic susceptibility) compared to the protolith. More detailed magnetic measurements were used to determine the magnetic mineralogy of each unit. The principal mechanism responsible for the high magnetic susceptibility values of the red gouge is the neoformation of magnetite from siderite or pyrrhotite caused by thermal pressurization during the latest earthquakes, which reveals the location of the latest seismic slip zone. By contrast, the magnetic susceptibility values of the black gouge have been reduced by the formation of goethite during weathering. We conclude that although the occurrence of goethite reveals paleo-earthquake, factors such as the oxidation or dissolution of fluids that effect magnetic minerals need further investigation.

Lithologic and rock magnetic evidence for the Mid-Miocene Climatic Optimum recorded in the sedimentary archive of the Xining Basin, NE Tibetan Plateau

Abstract Thick Cenozoic deposits in the Xining Basin provide great potential for understanding the tectonic and climatic evolution of the NE Tibetan Plateau. However, the Middle Miocene Climatic Optimum (MMCO) in this basin has not been reported up until now, due to the poor exposure of the uppermost strata covering this interval. In this study, a 194 m-deep core of flood–plain to channel–flood deposits was obtained from the Tashan flats above the Xiejia Section in the Xining Basin. Magnetostratigraphic dating of the core determines its age at ca. 18.5–13.5 Ma. Lithological and rock magnetic analyses demonstrate that the most prominent changes recorded in the present study occurred between ~ 17 and ~ 14 Ma. During this period, the sedimentary sequence mostly consists of gray-white or bluish-gray claystone and muddy siltstone, intercalated with some thin red claystone. The rock magnetic parameters show a large variability, with higher values in thin red claystone beds and lower values in gray-white or bluish-gray layers. These features demonstrate that, due to a warm and wet climate, extensive rivers had developed in the Mid-Miocene and that a reducing sedimentary environment prevailed. These “favorable” climatic conditions coincide with the MMCO as based on global deep-sea deposit records, indicating that this warm event exerted a major impact on regional climate change in the NE Tibetan Plateau during the Mid-Miocene.

Age and paleomagnetism of the 1210 Ma Gnowangerup–Fraser dyke swarm, Western Australia, and implications for late Mesoproterozoic paleogeography

Dolerite dykes of the Gnowangerup–Fraser Dyke Suite are subparallel to the southern and southeastern margins of the Yilgarn Craton. We collected oriented samples for paleomagnetic study from 19 dykes along the Phillips and Fitzgerald Rivers and north of Ravensthorpe. Alternating-field (AF) demagnetization revealed a stable two-polarity remanence in 14 dykes, and the primary nature of the magnetic directions is supported by a positive baked-contact test and by rock-magnetic evidence. U–Pb zircon and baddeleyite ages for two dykes confirm that the Gnowangerup–Fraser dykes are part of the 1210Ma Marnda Moorn Large Igneous Province. The mean paleomagnetic pole, at 55.8°N, 323.9°E, A95=6.5°, is almost identical to the previously reported VGP of the 1212Ma Fraser dyke, also supported by a positive baked-contact test. The combined robust paleopole places the West Australian Craton in a near-polar position at 1210Ma. Comparison with coeval Laurentian paleopoles indicates that Laurentia and Australia were widely separated at that time. We present a paleomagnetically permissible drift model for these two continents between 1210 and 1070Ma. One dyke yields a stable remanence with a VGP similar to the paleopole for the 755Ma Mundine Well dykes indicating that this dyke may have been emplaced during the same event at c. 755Ma. Differences in lengths and shapes of late Mesoproterozoic Apparent Polar Wander Paths of several continents suggests that a large supercontinent did not exist between about 1300 and 1050Ma. This may have been a transitional time between the final breakup of Nuna and the assembly of Rodinia.

Rock-magnetic evidence for the low-temperature emplacement of the Habushiura pyroclastic density current, Niijima Island, Japan

Abstract The emplacement temperatures of accidental and juvenile fragments in the Habushiura pyroclastic density current (PDC) deposit were estimated using analysis of rock magnetism. The Habushiura PDC was generated in a costal or shallow offshore area during the early stage of the AD 886 phreatomagmatic eruption on Niijima Island, Japan. We collected 160 samples from 11 beds, which include three lithofacies types: a massive lapilli tuff (Facies A); a graded and/or diffusely stratified lapilli tuff (Facies B); and a swarm of large pumice (Facies C). Juvenile specimens from Facies A and C show a wide range of emplacement temperatures, from less than 150 °C up to 300 °C, in contrast with the emplacement temperatures lower than 150 °C of Facies B. Morphological features of the ash components and random changes in palaeomagnetic direction by stepwise thermal demagnetization indicate that low-temperature emplacement occurred by phreatomagmatic eruptions and ingestion of ambient air by the turbulent current. The emplacement temperatures of accidental fragments were up to 380 °C, which is higher than juvenile fragments. They are thought to have cooled more slowly than juvenile fragments due to the smaller surface area with respect to their volume.

Rock magnetic evidence for inclination shallowing in the early Carboniferous Deer Lake Group red beds of western Newfoundland

SUMMARY A paleomagnetic and rock magnetic study of the Carboniferous Deer Lake Group red beds of Newfoundland was performed to detect and correct for inclination shallowing. Results indicate a primary remanence carried by magnetite, with a mean direction of D = 179.7 ◦ , I = 33.7 ◦ , α95 = 7.2 ◦ which corresponds to a paleopole position of 22.2 ◦ N, 122.3 ◦ E,A95 = 7.6 ◦ . Correcting the inclination using anisotropy of anhysteretic remanence and the measured individual particle anisotropy gives a corrected direction of D = 178.8 ◦ , I = 50.9 ◦ , α95 = 6.3 ◦ corresponding to a paleopole position at 8.4 ◦ N, 122.7 ◦ E, A95 = 7.2 ◦ . This correction is larger than that of other red beds from the Maritime Provinces of Canada, but is consistent with paleoenvironmental reconstructions, placing North America in a more arid climate zone. Our inclination-corrected results have important implications for this portion of North America’s apparent polar wander path and suggest a correction is needed for other red bed-derived APWPs. We have determined the range of flattening factors f , defined as the proportionality constant between the tangents of the measured (I m) and field (I o) inclinations, tan(I m) = f tan(I 0), from this study and previous inclination correction studies to estimate inclination corrections. Using the range of haematite f factors observed in this study to correct the Neogene red bed inclinations from the Vall` es-Pened` es Basin (NE Spain) yields inclinations consistent with the known geomagnetic field inclination in the Neogene, thus indicating that the range of f factors reported here may be used to estimate the magnitude of inclination shallowing in red beds.

An assessment of the reliability of palaeointensity results obtained from the Cretaceous aged Suhongtu section, Inner Mongolia, China

Here we present microwave palaeointensity results from 89 sister samples from the study of Zhu et al. [Zhu, R., Pan, Y., He, H., Qin, H., Ren, S., 2008. Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: implications for the field variation during the Cretaceous normal superchron. Phys. Earth Planet. Int., 169, 59–75] who carried out Thellier palaeointensity analysis as part of their integrated palaeomagnetic and 40Ar/ 39Ar dating study of Cretaceous lava from the Suhungtu section, Inner Mongolia, China. Additionally, a comprehensive rock magnetic investigation has been carried out in order to determine the mineralogy and hence the validity of assuming that the remanence is a thermal remanent magnetisation (TRM). The microwave results are of apparent high quality and give flow mean palaeointensity estimates ranging from 13 to 49 μT corresponding to virtual dipole moment (VDM) estimates ranging from 2.5 to 8.9 × 10 22 Am 2, and an overall mean VDM of 5.5 ± 1.9 × 10 22 Am 2 for the 24 flows (aged 110.6 ± 0.1 Ma). When the microwave results (using the perpendicular applied field method with partial microwave thermal remanence (pT MRM) and pT MRM tail checks) are compared to those obtained with the Thellier method (Coe version with pTRM but not tail checks, and heating in argon atmosphere) differences are seen at the sample, flow and palaeomagnetic unit level however, the overall means and spread in palaeointensity estimates are consistent. Some discrepancy is due to the differing sized sample sets and sample inhomogeneity but discrepancy is also interpreted to be due to the differing protocols, methodology, plus the subjectivity in interpretation. Considering only those results that are consistent to within 20% the spread in palaeointensity estimates remains. There is substantial rock magnetic evidence from progressive heating in air and argon experiments (both showing irreversible thermomagnetic behaviour) as well as looking at samples under the scanning electron microscope to suggest that maghaemite is present (albeit to varying degrees) in many of the samples. Alteration therefore occurred in nature and it is likely that the remanence will have been affected to differing degrees potentially causing underestimates in palaeointensity. Maghaemite is interpreted to be a remanence carrier where a component of remanence remains after heating to 580 °C and loss of pTRM acquisition capacity is found on heating. No correlation was found between the estimated palaeointensity (microwave or Thellier datasets) and amount of high temperature remanence or any rock magnetic parameter. This could suggest that the palaeointensity estimates are reliable or as seems likely more than one factor (such as methodology, protocol, interpretation, chemical remanent magnetisation (CRM) contamination and the geomagnetic field) are influencing the palaeointensity estimates. In an attempt to remove the influence of palaeointensity protocol and methodology the palaeointensity datasets were reduced to consist only of those results mutually consistent to 20% ( N = 22). No obvious correlation was again seen between the inferred maghaemite contribution and palaeointensity estimate. There is therefore no clear evidence for the biasing of results due to CRM contamination (assuming a simple relationship between biasing and maghaemite contribution) and it can thus be inferred that the geomagnetic field was a major influencing factor. Further studies are however needed to fully elucidate the extent and result of each possible influencing factor and ultimately the reliability of palaeointensity estimates from rocks of this type.

Rock magnetic evidence for inclination shallowing in the Passaic Formation red beds from the Newark basin and a systematic bias of the Late Triassic apparent polar wander path for North America

The latest Triassic and earliest Jurassic red beds from the Newark basin possess paleomagnetic inclinations that are ∼ 10° shallower than the inclinations carried by coeval volcanic rocks. It has been suggested that the difference might be due either to inclination shallowing of the sediments or to remagnetization of the volcanic rocks during a Middle Jurassic hydrothermal event. Either explanation would in turn yield significantly different apparent polar wander paths (APWPs) for the Late Triassic and Early Jurassic of North America. Here we present paleomagnetic and magnetic anisotropy data for the latest Triassic Passaic Formation red beds, which reveal deposition and/or compaction fabrics with 10% anisotropy of the high unblocking temperature, reversed polarity remanent magnetization. Inclination correction for the red beds yields similar directions to that carried by the earliest Jurassic volcanic rocks, indicating that inclination shallowing is the major cause for the difference in paleomagnetic pole position between the red beds and the volcanic rocks. Comparison of the inclination corrected data with the Late Triassic North American APWP shows a systematic error in the APWP, likely due to inclination shallowing. In turn, this error may be even more significant for much of the Paleozoic part of the North American APWP.

Paleomagnetic and rock magnetic evidence for a secondary yet early magnetization in large sandstone pipes and host Late Middle Jurassic (Callovian) Summerville Formation and Bluff Sandstone near Mesita, west central New Mexico

Processes responsible for the acquisition of ancient yet secondary magnetizations are important facets of the geologic history of rocks and, when the age of such magnetizations can be estimated with confidence, provide useful information on the ancient geomagnetic field. In west central New Mexico near Mesita, on the Colorado Plateau, hematitic sandstone and siltstone beds of the Middle Jurassic (Callovian) Summerville Formation and overlying Bluff Sandstone are host to numerous large (up to 100 m2 in map area) pipe‐like sandstone bodies. The pipes are as strongly cemented by hematite (colors range from 10R 6/6 to 10R 3/4) as the host strata; paleomagnetic data from them and their host strata are interpreted to indicate that these rocks have been remagnetized, probably in association with sandstone pipe formation. Reverse polarity magnetizations isolated in both alternating field and thermal demagnetization from pipes are well grouped and are similar to, and not statistically distinct from, those in adjacent host strata. The grand‐mean direction for 16 sites (7 sites in sandstone pipes and 9 in host strata), corrected for slight (5°) west‐northwest tilt of the strata, is D = 163.0°, I = −44.3° (α95 = 2.7°, k = 169). This direction yields a pole position of 72.8°N, 135.7°E (dp = 2.1°, dm = 3.4°). Assuming a modest (i.e., ∼5°) clockwise rotation of the Colorado Plateau, the pole lies at 68.7°N, 143.8°E. Median destructive fields for the remanence in pipes and host strata are typically 40–50 mT; over 90% of the remanence is “unblocked” or removed during changes in the magnetic mineralogy by temperatures of ∼400–450°C. Isothermal remanent magnetization (IRM) acquisition data, and thermal demagnetization of “saturation” IRM, however, demonstrate that the dominant magnetic phase is of high coercivity and relatively high (above 600°C) laboratory unblocking temperatures in both sandstone pipes and host strata, yet it does not appear to contribute significantly to the characteristic remanent magnetization. The similarity in demagnetization properties between pipes and adjacent host strata, the absence of a well‐defined high unblocking temperature remanence that is more typical of hematite‐cemented detrital strata, and the essentially uniform reverse polarity of the remanence are all interpreted to indicate that pipes and host strata contain secondary, yet early acquired magnetizations and that magnetization acquisition continued after pipe injection. We propose that acquisition of the secondary magnetization took place in the presence of alkaline, high pH brines formed by the dissolution of the underlying gypsum‐dominated Lower Jurassic Todilto Formation strata and therefore the remanence is early in age. On the basis of a comparison with Summerville and Morrison (Middle and Late Jurassic) paleomagnetic poles from rocks on the Colorado Plateau, we interpret the secondary remanence in Summerville strata and sandstone pipes near Laguna to be latest Middle to Late Jurassic in age. If realistic, this interpretation further emphasizes the importance of fluid‐rock interaction in the acquisition of secondary magnetizations.

Rock magnetic evidence of inflation of a flood basalt lava flow

The anisotropy of magnetic susceptibility (AMS) of lava flows is an innovative method which has been proved to be directly related to the shear history of lava. One of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows. This feature of the AMS method makes it very attractive to gain insight into the mechanism of emplacement of massive, relatively featureless, long lava flows such as those forming flood basalt provinces. In this work, we report the results of the measurement of AMS as a function of vertical position within the Birkett lava flow, one of the Columbia River Basalt Group flows. The observed variation of AMS allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events. The AMS-estimated thickness of each injection event (in the range of 0.5–4.0 m) coincides with the range inferred for injected lava pulses in modern Hawaiian lava flows. Thus, the evidence provided by the AMS method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present-day inflated pahoehoe flows. Regarding the orientation of the principal susceptibilities, in the central part of the flow they define a preferred orientation along an E–W trend, whereas in the outer parts of the flow they have a NNE–SSW trend. This difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-002-0203-8.

Evaluating the timing of hydrocarbon generation in the Devonian Duvernay Formation: paleomagnetic, rock magnetic and geochemical evidence

The physical and chemical changes associated with the thermal maturation of organic-rich shale have affected the paleomagnetic and rock magnetic characteristics of the Devonian Duvernay Formation in the Western Canada Sedimentary Basin. This formation has several lithofacies that correspond to deposition in platform, slope and deeper water settings under varied redox conditions. Shale, laminated mudstone and some massive mudstone facies show evidence of magnetic changes associated with maturation but wackestone, packstone and some massive mudstone facies appear to be unaffected by the process. Rock magnetic evidence suggests that thermal maturation induces a change in the magnetization carrier from magnetite and hematite to solely magnetite.The packstone and wackestone facies commonly show a reversed characteristic magnetization with a paleopole at 194°E, 70°N (A95=13.2) of Late Cretaceous-age. Shale and laminated mudstone facies in immature areas of the basin have inclination-only characteristic remanent magnetization (ChRM) means that range from 55 to 67°C, requiring a pre-Cretaceous magnetization age. Shale and laminated mudstone facies in mature areas of the basin have a much steeper ChRM in direction ranging from 77 to 83°C. Their very steep nature suggests that step demagnetization has not completely removed a drilling-induced remanence in some wells.

Opaque mineralogy and magnetic properties of selected banded iron‐formations, Hamersley Basin, Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.

Palaeomagnetic results from remagnetized mid-Cretaceous(Albian-Cenomanian) strata of northeastern Venezuela

SUMMARY We report palaeomagnetic and rock magnetic results of a sedimentary sequence (Pertigalete cement quarry) located in northeastern Venezuela. Sampling was restricted to the vicinity of the contact between the upper Cretaceous Chimana and Querecual formations. Biostratigraphic evidence reveals an upper Albian age for this formational transition. Profiles of site-averaged NRM intensity of the high-coercivity (over 30 mT) and high-temperature (over 400°C) components appear to be related to the contact and distance from the contact. We interpret this profile as the probable outcome of overlapping thermochemical remagnetization events resulting from hydrothermal activity that was focused along the two formations. Direct spectral analyses performed on the site-averaged stable NRM intensity profile allow the separation of at least two of these remagnetization events. On the other hand, palaeomagnetic results show a considerable streaking of site mean declinations, suggesting that tectonic or structural horizontal movements around a vertical axis have occurred after NRM acquisitions. Horizontal rotation angles, plotted against stratigraphic levels for bedding-corrected data, show some features that seem to coincide with the alteration peaks isolated in the profile of site-averaged stable NRM intensities. Thus, it appears that repeated thermochemical remagnetizations with overlapping unblocking spectra, and horizontal movements around a vertical axis could have been responsible for much of the within-site dispersion. A simple three-stage reconstruction of the possible chain of thermochemical and tectonic occurrences that could lead to the present-day palaeomagnetic and rock magnetic evidence is proposed. These events, including clockwise horizontal rotations around a vertical axis, are tentatively placed in a geological time framework between middle Miocene and Pliocene times according to the main geological and geochemical evidence available.

Problems of core correlation, sediment source ascription and yield estimation in Ponsonby Tarn, West Cumbria, UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from 210Pb and 134Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a−1 and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a−1. These represent yields of 7·0 t km−2 a−1 and 39·8 t km−2 a−1, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.

Problems of core correlation, sediment source ascription and yield estimation in Ponsonby Tarn, West Cumbria, UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from 210Pb and 134Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a−1 and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a−1. These represent yields of 7·0 t km−2 a−1 and 39·8 t km−2 a−1, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.

Paleomagnetic and rock magnetic evidence for inverse zoning in the Parguaza batholith (southwestern Venezuela) and its implications about tectonics of the Guyana shield

We report paleomagnetic and rock magnetic data from the rapakivi granites of the Parguaza batholith (Guyana Precambrian Shield, southwestern Venezuela). These results suggest that the pluton is inversely zoned with respect to the cooling ages. In order to explain such an age pattern, tentative structural settings are proposed placing the Parguaza intrusions in a plate tectonic context. Six sites were sampled along a 200 km transect that cuts through the northern lobe of the batholith. Thermomagnetic curves, X-ray diffraction and fluorescence, hysteresis loops, thermal and alternating field (AF) intensity plots, transmission (TEM) and scanning (SEM) electron microscope analyses and Königsberger ratios (Qn) values were used to identify the different magnetic mineralogies and their distribution of grain sizes. Magnetite, titanomagnetite near magnetite in composition and deuteric hematite are the three carriers of natural remanent magnetizations (NRMs) in these rocks. Magnetic granulometry indicators such as Königsberger ratios (Qn values) suggest the dominant presence of single domain magnetites with average grain sizes grading from finer to coarser away from the center of the transect. The paleomagnetic results reveal the existence of two primary thermoremanent and/or thermochemical magnetizations (TRMsTCRMs) for sites CSP-3, PI-2 and PI-4 (Decl. = 328°, Incl. = −21°, k = 15, α95 = 11.4°) and site CSP-2, PI-1 and PI-3 (Decl. = 16°, Incl. = 87°, k = 10, α95 = 13°), respectively. There is also a poorly defined G3R2 magnetization (Decl. = 284°, Incl. = −86°, k = 6, α95 = 27°) found in sites CSP-2, PI-1 and PI-3. The overlap of coercivities and the unblocking temperatures spectra, probably resulting from the coexistence of primary single-domain magnetic mineralogies with secondary exsolutions of single-domain-like Ti-poor (Fe-rich) regions (almost pure magnetite) in multidomain titanomagnetite grains, in most cases precludes the complete resolution of hybrid G1 (N + R) or G3 (N + R). The relative ages for these components were determined using a map of RbSr model age ‘chrontours’. G1 and G3 are the older and the younger TRMsTCRMs, respectively, and were acquired at two discrete moments of the batholith's geological history. The final map resulting from integrating the paleomagnetic and RbSr data, shows an age pattern for this batholith, and the rest of the intrusions that belong to the Parguaza Igneous Complex, that could be explained either as the effect of inverse cooling of a single intrusive body, the sequential emplacement of magmas controlled by normal faulting or an internal tectonism resulting in a system of NE-trending horsts and grabens cutting through the pluton. Because of its feasibility and agreement with the most recent theories about the tectonic evolution of the Guyana Shield, we favor the latter hypothesis.

Equatorial extension of the younger Dryas event: rock magnetic evidence from Lake Magadi (Kenya)

Magnetic parameters measurements (magnetic susceptibility χ and Isothermal Remanent Magnetizations (IRMs) and sedimentological analyses have been performed on Late Pleistocene/Holocene laminated deposits from Lakes Natron and Magadi (Kenya). Accelerator Mass Spectrometry (AMS) 14C dating on the Total Organic Matter (TOM) locates a continuous laminated sequence in the 12-10 ka B.P. time interval. The results show the occurrence of a two-stepped lacustrine optimum. A first humid period, resulting in a high-stand above a 635 m altitude geographical barrier, reached its maximum between 12 and 11 ka B.P. Between 11 and 10.7 ka B.P., the lake level dropped below the 635 m altitude barrier, which resulted in the separation of Lake Natron and Lake Magadi. A second maximal high stand period occurred from ca. 10 ka B.P., at the beginning of Holocene times. The 11-10 ka arid event recorded in Lake Magadi confirms previous studies which suggested an equatorial extension of the Younger Dryas event. Magnetic parameters and microfacies analyses suggest that this event was produced by a progressive and general weakening of monsoonal rainfall in East Africa.

Thermal alteration of the magnetic mineralogy in ferruginous rocks

The Northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite. The magnetic fabric of this sedimentary rock, as defined by the anisotropy of magnetic susceptibility, is controlled by bedding compaction but is inverse (i.e., the maximum susceptibility axes are normal to the bedding plane). This inverse fabric is attributed to the dominating presence of siderite. The rocks were incrementally heated to 600°C, and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step; cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric. After heating the samples above 250°C, the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane. The low‐temperature magnetic fabric remains inverse until the samples are heated to about 500°C. After heating to higher temperatures a normal fabric was observed at both room temperature and low temperature. Rock magnetic evidence shows that magnetite is created during the heating. The start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy, powder X ray diffraction, and acquisition of isothermal remanence. Berthierine starts to break down at approximately 250°C and is totally oxidized by 500°C. The creation of magnetite in the heating experiments corresponds with this mineralogical change. The change in the room temperature magnetic fabric is associated with the decomposition of berthierine, part of which alters to magnetite. The new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite.

Rock magnetic evidence for the dissolution and authigenic growth of magnetic minerals within anoxic marine sediments of the California continental borderland

The rock magnetic properties of marine sediments from the California continental borderland (San Pedro, Santa Catalina, and San Nicolas basins) have been studied in order to quantitatively assess the effects of sediment diagenesis on magnetic minerals. Previous studies have noted that the natural remanent magnetization in these sediments, primarily carried by detrital magnetite, decays to 10% or less of its surface value soon after deposition. This decrease is caused by magnetite dissolution related to sediment diagenesis and is unrelated to paleoclimatic variations or changes in the regional influx of detrital magnetic material. Detailed rock magnetic measurements show that shifts to softer remanent coercivity and differences in the rate and degree of magnetic intensity loss with depth can be related to the dissolution process. The shift to softer remanent coercivity is related to a coarsening of the magnetic mineral grain sizes with depth due to preferential dissolution of the finest‐grained magnetic material. The intensity decreases, which are linearly proportional to magnetite concentration decreases, indicate that dissolution occurs with rate constants ranging from 0.3 to 1.6 kyr−1. The rate constants, sulfide concentrations, and magnetite grain size estimates from the borderland are consistant with previous studies of magnetite dissolution. Our results demonstrate the importance of both sulfide and magnetite surface area in the dissolution process. Anomalous peaks in viscous remanence within the sediments suggest the authigenic growth of greigite and its subsequent transformation to pyrite.