Normal Polarity Interval Research Articles

Paleomagnetism of the Ediacaran Avellaneda Formation (Argentina), Part II: Magnetic and chemical stratigraphy constraints on the onset of the Shuram carbon excursion

Magnetostratigraphy is a powerful technique for high-resolution stratigraphic correlation and accurate dating of the polarity history of the magnetic field. Up to now, magnetostratigraphic studies have been underutilized in Ediacaran successions. Here, we present a magnetostratigraphy coupled with a carbon isotope study in Ediacaran Avellaneda Formation (∼570 Ma), a sedimentary cover sequence in the Tandilia orogenic belt, of the Río de la Plata craton. Paleomagnetic results and δ13C profiles were obtained from three drill cores that cut across the sedimentary record of the Avellaneda Formation. Paleomagnetic results plotted stratigraphically allowed the identification of a prevalent reverse polarity separated by normal polarity interval. Our findings show a coincident correlative polarity transition at each drill core analyzed. δ13C values demonstrate that the magnetostratigraphic polarity pattern is laterally consistent. Furthermore, C-isotope curves show a transition from positive to negative values near the top of the Avellaneda Formation. This excursion is likely correlative with the onset of the Shuram excursion (ca. 570 Ma). Our findings suggest that the onset of Shuram excursion took place during a reverse polarity chron. Comparison with other coeval sequences validate this correlation. Geomagnetic polarity reversals pattern in the Avellaneda Formation and its correlative pattern, validates a magnetostratigraphy as a potential method to correlate Precambrian strata. Our new findings shed further light on the possible use of magnetic polarity reversals to correlate Ediacaran successions.

Paleomagnetism of Tarkhanian sediments of the section Skelya (Kerch Peninsula)

With the goal to correlate the magnetostratigraphic data of the Eastern Paratethys obtained by researchers in different years a rockmagnetic and paleomagnetic investigations of the Tarkhanian sediments of the Skelya section was carried out. The main carrier of the natural remanent magnetization is titanomagnetite in the Argunian beds of the Tarkhanian regional stage; in addition to titanomagnetite there is iron sulfide (monoclinic pyrrhotite and/or greigite) in the deposits of the Terskian and Kuvinian beds. The paleomagnetic fold test of natural remanent magnetization directions, recorded in the rocks of the Argunian beds, is positive, which indicates a pre-folding age of the magnetization formation. A comparison with the magnetochronological scale GTS 2020 shows that the normal polarity interval in the lower part of the Argunian beds can presumably be compared with the С5Bn chron. The reversal polarity interval in the upper part of the Argunian beds is with C5ADr. The age of the С5Bn/C5ADr boundary can be estimated as ~14.7 Ma.

New chronological constraints for the lowermost stratigraphic unit of Atapuerca Gran Dolina (Burgos, N Spain)

We present the results of a combined Electron Spin Resonance (ESR) and Luminescence dating study for the lowermost stratigraphic unit (TD1) of the palaeoanthropological site of Atapuerca Gran Dolina, Spain. Ten samples collected through the Main Section were dated using either the Multiple Centre (MC) ESR approach or the single-grain thermally transferred optically stimulated luminescence (SG TT-OSL) technique, both of which were applied to quartz grains. The two methods yield consistent ages, providing a robust chronostratigraphic framework for the TD1 deposits and enabling improved correlation of the magnetostratigraphic succession (comprising three intervals of normal polarity; N1 to N3, from bottom to top) with the Geomagnetic Polarity Time Scale. Specifically, the results provide a late Early Pleistocene chronology for TD1, and clearly position the deposits in the post-Olduvai time range (<1.77 Ma). The final age estimates range from 1.30 ± 0.14 Ma (TT-OSL) close to the bottom of the section to a mean ESR age of 1.05 ± 0.12 Ma (1 s.d.) for three samples from the N3 interval towards the top. Consequently, the N3 interval may confidently be correlated to the Jaramillo Subchron, while we propose a correlation of N1 and N2 with the Cobb Mountain subchron and Punaruu excursion, respectively; although we cannot exclude that these two normal intervals may correspond to other, unknown, geomagnetic events given the existing uncertainties associated with the ESR and luminescence ages. From a methodological point of view, the ESR MC approach shows that the Titanium signal provides the most reliable burial ages for TD1. In contrast, the Aluminium centre yields systematically overestimated age estimates, as a result of incomplete resetting of the signal before sediment deposition. The SG TT-OSL ages presented here are amongst the oldest published so far, and highlight the potential of the high temperature SG TT-OSL 290 approach for obtaining finite ages exceeding 1 Ma at Atapuerca. Finally, these new dating results provide key insights into the rates and nature of sedimentary processes at Gran Dolina, modifying long-standing interpretations of the cave formation and infilling dynamics. An age of at least 1.4 Ma may be inferred for the base of the sedimentary infill at the Main Section, providing minimum age constraint for the formation of the cavity and, by extension, for the intermediate level of the Atapuerca karst. The opening of the cave, which pre-dates human occupation, most likely occurred close to or slightly after 0.9 Ma. This suggests that both TD4 and TD6 archaeological levels were deposited within a relatively short time range of <100 kyr. The so-called autochthonous fluvial deposits from TD1 unit, or at least those dated in the present study, were also clearly primarily derived from outside the karst system, and were not reworked from older deposits within the karst. They experienced sufficient daylight exposure and minimum transport times within the karst system to enable consistency between the ESR and TT-OSL burial ages and the independent magnetostratigraphic evidence. Our results highlight the need to reconsider the terminology traditionally employed to differentiate the two main phases of Gran Dolina's sedimentary infill. New numerical age results for stratigraphic unit TD1 of Atapuerca Gran Dolina, Spain The presence of Jaramillo magnetic Subchron (1.07-0.99 Ma) is confirmed The base of the sedimentary infill post-dates Oduvai Subchron (<1.77 Ma) Cave opening occurred ca. 0.9 Ma ago, suggesting that TD4-6 units deposited in < 100 kyr TD1 fluvial deposits do not originate from within the karst

Paleomagnetism of the Ordovician section of the Mishina Gora (Pskov oblast): a record of the ancient geomagnetic field and restrictions on the age of the Mishinogorsk explosive structure

Paleomagnetic studies of the Ordovician rocks of the Mishina Gora explosive structure (Gdovsky district, Pskov region) have been carried out. Based on the results of thermal magnetic cleaning, two stable magnetization components S and P have been identified. These components have similar unblocking temperature ranges (~200–600°C), but their mean directions are statistically different the difference of their mean directions are statistically significant. Refional fold–test indicates the postfolding age of the component S and prefolding age of the component P. Prefolding age of the latter is supported by the positive result of the local fold-test which has been carried out in the scale of the studied section. Comparison of the paleomagnetic pole calculated from the S component with the Apparent Polar Wander Path for the East European Platform definitely indicates the Late Permian-Early Triassic age of this component, which, in combination with the available geological observations, determines the time range of the formation of the Mishina Gora structure as ~370-250 million years ago. The ancient component of the magnetization P has the same direction as the components, which have been identified earlier in a number of coeval sections of the North-West of Russia, Estonia, and Scandinavia and which have been considered to be primary. The corresponding paleomagnetic poles are widely used in Paleozoic geodynamic and tectonic paleoreconstructions with the participation of the Baltic paleocontinent. A positive fold test result for the P component validates the use of these poles for paleoreconstructions. For the stratigraphic interval, which includes the upper Floian, Dapinian, and lower 2/3 of the Darrivilian stages, a magnetostratigraphic section has been constructed containing a single magnetozone corresponding to the reverse polarity of the geomagnetic field. Thus, the magnetostratigraphic data do not support the hypothesis of the existence of intervals of normal polarity within the Ordovician geomagnetic superchron of reverse polarity (Moyero superchron).

U-Pb zircon age constraints on the vertebrate assemblages and palaeomagnetic record of the Guadalupian Abrahamskraal Formation, Karoo Basin, South Africa

The rich fossil record of the Permo-Triassic Beaufort Group of the main Karoo Basin serves as a reference sequence for tetrapod biochronology and provides a unique window into the evolution of terrestrial ecosystems between the Guadalupian and Middle Triassic. Absolute temporal constraints on the Beaufort Group have improved in the last decade with the application of U-Pb zircon geochronology but precise ages have not yet been reported for its basal Abrahamskraal Formation. Consequently, the age of the Eodicynodon Assemblage Zone and the lower boundary of the Tapinocephalus Assemblage Zone have remained less clear than younger Permian biozones. An improvement in temporal constraints also has implications for the correlation of palaeomagnetic logs reported from the Abrahamskraal Formation. In this paper we present new CA-ID-TIMS U-Pb geochronology from tuff beds that provide the first high-resolution chronostratigraphy for the upper two thirds of Abrahamskraal Formation. These ages constrain the base of the Tapinocephalus Assemblage Zone to be older than 264.4 Ma, and closely constrain the boundary between its two subzones to ca. 262 Ma. Combined with published ages from the overlying Teekloof Formation, our data show the Tapinocephalus Assemblage Zone is latest Wordian to late Capitanian in age. These complement previously published CA-ID-TIMS U-Pb ages for younger Beaufort rocks and suggest that the normal polarity interval observed in the middle Abrahamskraal Formation represents a normal (probably third) chron within the mixed-polarity Illawarra Superchron.

Correlation of Eemian sections in Lithuania and Belarus based on palaeomagnetic, radioisotope and palaeobotanic data

Eemian (Murava, Merkinė deposits at five exposed sections (Zaslavl, Zhukevichi, Ponemun, Snaigupėlė, and Netiesos) located in Lithuania and Belarus are described. Preliminary palaeomagnetic results show a record of the Brunhes epoch normal magnetic field and a short-term reversal – the Blake Event – is recognized in three of the five sections. The Blake Event recorded in the Netiesos section is characterized by a pattern consisting of three short reversed polarity intervals separated by two short normal polarity intervals. The directional changes of declination, inclination, and MS (magnetic susceptibility) are clear. ESR dating (112.5 ±10.8 and 112.1 ±25.9) and 230Th/U dates obtained from this section (108.8 ±12.0/9.9 ka for the L/L technique and 100.2 ±10.3/8.6 ka for the TSD technique) suggest that (Blake and post-Blake) palaeomagnetic excursions are present in this section. Palaeobotanical analysis and isotope dating of the Netiesos section suggest that the Blake Event occurred during the climatic optimum.

Relative Paleointensity Record of Integrated Ocean Drilling Program Site U1396 in the Caribbean Sea: Geomagnetic and Chronostratigraphic Observations in the Pliocene

AbstractContinuous sedimentary records of paleomagnetic directional variability and relative paleointensity (RPI) provide valuable information on the evolution of the geodynamo while also facilitating stratigraphic correlation and age control. While the Quaternary RPI record has received much attention, Pliocene records are relatively rare. Here, a u‐channel paleomagnetic study from Integrated Ocean Drilling Program (IODP) Site U1396 in the Caribbean Sea refines the shipboard‐derived polarity stratigraphy and generates an RPI and directional record extending back 4.5 Ma. Rock magnetic data reveal changes in magnetic coercivity around 2.1 Ma that influences the quality of the paleomagnetic record; while the older record is well‐resolved and passes RPI quality criteria, much of the younger section does not. To facilitate the development of the RPI record, spike noise associated with discrete intervals of volcanogenic sediments are filtered from the data set. The resulting record passes RPI reliability criteria between 0–0.6 Ma and 2.1–4.5 Ma and represents the highest resolution RPI record extending into the early Pliocene. We use this record to refine the existing benthic δ18O chronology and open the door to high‐resolution RPI chronostratigraphies during the Pliocene. Although we find no evidence for a previously observed increase in magnetic field intensity after ∼4 million years ago, we do observe an asymmetrical form to RPI in the normal polarity intervals of the Gauss chron. We also find a polarity bias in inclination that cannot be simply explained by a drill string overprint, suggesting polarity‐driven field asymmetries that are particularly pronounced during the Pliocene.

Geomagnetic polarity during the early Silurian: The first magnetostratigraphy of the Llandovery

Magnetostratigraphic studies in the Silurian are absent, and what is understood about the geomagnetic polarity during this time is based on polarity bias-type data from palaeopole-type studies. We provide the first composite magnetic polarity record through the Lower Silurian (Llandovery) from the magnetostratigraphy of six sections. These are integrated with graptolite biostratigraphy and some carbon isotope chemostratigraphy. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured mudstones and mostly magnetite in non-red lithologies. The influence of possible tectonic disruption of the fabric is assessed using anisotropy of magnetic susceptibility. Only the most thermally mature section at Backside Beck shows the imprint of initial tectonic fabric formation. The Llandovery is divided into 6 major normal-reverse-polarity chron couplets (referred to as LL1 to LL6). An additional longer, exclusively normal polarity interval (referred to as WE1n), beginning in Telychian Stage slice Te3, runs into the lower Sheinwoodian. Within these five polarity couplets there are 10 further submagnetozones, and 10 tentative submagnetozones. Average reversal frequency (including the tentative submagnetozones) was ca. 3.0 Myr−1 in the Early Silurian, which is probably an underestimate, due to insufficient sampling density in some parts of the Rhuddanian and Aeronian. This reversal frequency is similar to that in the late Cenozoic, indicating the future potential utility of magnetostratigraphy for high-resolution correlation and dating in the Early Silurian.

Middle–Late Permian magnetostratigraphy and the onset of the Illawarra Reversals in the northeastern Parana Basin, South America

SUMMARYRecent compilations of the Illawarra Reversals show remarkable differences in the onset age (265 Ma versus 269 Ma) and the magnetic polarity pattern, hampering the establishment of a reference geomagnetic timescale for the Middle–Late Permian. The Parana Basin hosts a 7-km-thick, discontinuous sedimentary succession spanning the Palaeozoic to the Mesozoic, including the Gondwana 1 supersequence which extends from Late Carboniferous to the Triassic or Jurassic. We provide the magnetostratigraphy of the middle and upper Permian part of this sequence of a ca. 300-m-long core, pertaining to the Passa Dois Group and the Piramboia Formation. Sample demagnetization of NRM revealed two magnetic overprints ascribed to the Brunhes chron and to the emplacement of Parana Large Igneous Province in the Early Cretaceous, respectively. Stable, dual polarity characteristic components (ChRM) were isolated at temperatures higher than 450 °C and interpreted as a primary detrital magnetic remanence (DRM), acquired during or soon after sediment deposition. Available U-Pb dating on volcanic zircons from the literature provided independent chronologic constraints for our magnetostratigraphy. A total of 8 reverse polarity intervals were identified, the lowermost of which (up to 110 m thick) correlates to the Kiaman Superchron. The overlying Illawarra is dominated by a reverse polarity magnetization with thin (4–5 m thick) normal polarity intervals. This pattern agrees with the Illawarra sequence from the Karoo Basin and other proposed timescales for the Permian. The onset of the Illawarra reversals is interpolated to ca. 270 Ma, close to the age of 269 Ma from the Karoo Basin. Combined magnetostratigraphy and geochronology yielded sediment accumulation rates of ca. 8 m Myr–1 for the Passa Dois Group in the northeastern (marginal) sector of the Parana Basin, indicating that Serra Alta and Teresina Formations span 279–274 Ma and 274–254 Ma, respectively.

A robust geochronology of the Yangtze River Delta based on magnetostratigraphy and cyclostratigraphy of sediment core ZKA2

Studying the evolution of sea-level fluctuations, climate oscillations and tectonic movements in the Yangtze River Delta requires a reliable chronology. Here, we present the results of a comprehensive study of the magnetostratigraphy and cyclostratigraphy of a 295-m-long sediment core (ZKA2) recovered from Nantong City, Jiangsu Province, China. Anisotropy of magnetic susceptibility (AMS) studies reveal that the petrofabrics are mainly primary. Based on rock magnetic and thermal demagnetization analyses, the major magnetic minerals in the studied sediments are magnetite, hematite and greigite, but only magnetite and hematite carry the primary remanence. The magnetostratigraphy reveals seven reversed polarity intervals, nine normal polarity intervals, and one interval of mixed polarity. Variations in natural gamma-ray activity reflect variable continental weathering in the sediment source regions and are paced by Earth orbital cycles. Notably, a prominent 405-kyr long eccentricity cycle and ~100-kyr short eccentricity cycle are evident, which allow us to constrain the basal age of core ZKA2 to ~3.1 Ma. The magnetostratigraphic and cyclostratigraphic results together enable us to define the depths of key chronostratigraphic and magnetostratigraphic boundaries. They include the Late/Middle Pleistocene boundary at ~18-m depth, the Brunhes/Matuyama boundary at ~88-m depth, and the Matuyama/Gauss boundary at ~254-m depth. According the new chronology, we find that although clear long eccentricity (~405 kyr) and short eccentricity cycles (~100-kyr) cycles are evident in the magnetic susceptibility record, they are not well matched with cycles of natural gamma-ray activity. This suggests the source and post depositional process of the magnetic particles is complicated, and it should be cautious to choose magnetic properties for cyclostratigraphic analysis.

Magnetostratigraphy of a Middle Jurassic delta system (Lajas Formation), Portada Covunco section, southern Neuquén Basin, Argentina

A systematic paleomagnetic study was carried out in the Lajas Formation (Middle Jurassic), cropping out at the Covunco creek in the central part of the Neuquén basin. The Lajas Formation at the Portada Covunco section consists of a succession of conglomerates, sandstones and mudstones that reaches c. 290 m-thick. On the basis of lithological and textural features, sedimentary structures, strata geometries, and fossil content, 14 facies were defined that were grouped in two facies associations within a fluvial-dominated delta system. The age of the Lajas Formation is rather controversial, and has been assigned from sequence stratigraphy and palynological data. Thus from south to north, interpreted ages span the Upper Aalenian–Middle Bathonian through the Callovian. Therefore, magnetostratigraphy was used to better precise the age of the Formation at Portada Covunco. Paleomagnetic data obtained from 29 sampling sites present reliable behaviors. Rock magnetic studies (thermomagnetic curves, K vs H curves, IRM acquisition curves) suggest that the remanence is carried by Ti-poor magnetite. Anisotropy of magnetic susceptibility data show clustering of the Kmax axes produced by the action of paleocurrents. The characteristic remanent magnetization passed a tilt test and a reversal test that prove its primary origin. The resultant magnetostratigraphy is characterized by a reverse-dominant with minor normal polarity intervals. When compared with the Bajocian-Bathonian reference geomagnetic polarity time scale (GTS2016), the best fit is achieved in the Lower through Middle Bathonian (Chrons M41 to M39). A paleomagnetic pole was calculated for Portada Covunco located at 167.1°E, 59.4°S, N = 28, A95 = 3.6°, K = 57.2. When compared with the other available paleopole of the same age −170 Ma-derived from dykes exposed at Mamil Choique, Patagonia, Portada Covunco falls in the same paleomeridian although at a lower latitude. Thus, it is interpreted that the studied section was not subjected to rotations around vertical axis (R = 4° ± 10°), but was compacted, which produced inclination shallowing of the magnetic vector (F = 18° ± 11°). The calculated inclination shallowing and amount of compaction in the section are well within the expected values in medium to fine-grained sandstones. Using the paleomagnetic pole from Mamil Choique as reference, the locality of Portada Covunco would have been located at c. 33°S during the Bathonian, in agreement with paleoclimatic interpretations derived from palynological data.

The shutdown of an anoxic giant: Magnetostratigraphic dating of the end of the Maikop Sea

Paratethys, the lost sea of central Eurasia, was an anoxic giant during Oligocene – early Miocene (Maikop Series) times. With a size matching the modern-day Mediterranean Sea and a history of anoxic conditions that lasted for over 20 Myrs, the eastern part of this realm (Black Sea-Caspian Sea domain) holds key records for understanding the build-up, maintenance and collapse of anoxia in marginal seas. Here, we show that the collapse of anoxic Maikop conditions was caused by middle Miocene paleogeographic changes in the Paratethys gateway configuration, when a mid-Langhian (Badenian-Tarkhanian) transgression flooded and oxygenated the Eastern Paratethys. We present an integrated magneto-biostratigraphic framework for the early Middle Miocene (Tarkhanian-Chokrakian-Karaganian regional stages) of the Eastern Paratethys and date the lithological transition from anoxic black shales of the Maikop Series to fossiliferous marine marls and limestones of the regional Tarkhanian stage. For this purpose, we selected two long and time-equivalent sedimentary successions, exposed along the Belaya and the Pshekha rivers, in the Maikop type area in Ciscaucasia (southern Russia). We show that a significant but short marine incursion took place during the Tarkhanian, ending the long-lasting Maikop anoxia of the basin. Our magnetostratigraphic results reveal coherent polarity patterns, which allow a straightforward correlation with the time interval 15–12 Ma of the Geomagnetic Polarity Time Scale. The Tarkhanian flooding occurred during a relatively short normal polarity interval that correlates with C5Bn, resulting in an age of 14.85 Ma. The regional Tarkhanian/Chokrakian stage boundary is located within C5ADn at an age of 14.75 Ma and the Chokrakian/Karaganian boundary is tentatively correlated with C5ACn and an age of 13.9–13.8 Ma. Our new Tarkhanian flooding age reveals a paleogeographic scenario that is different from many previous reconstructions. Instead of envisaging marine connections to the Indian Ocean, we show that major changes in connectivity between the Eastern and Central Paratethys seas have caused the influx of marine waters during the Tarkhanian. An increase in marine connectivity with the Mediterranean during a short episode of rapid sea-level rise triggered mixing and ended the widespread anoxia in the Eastern Paratethys. The mixing episode was short-lived (~100 kyr) as the sea-level rise slowed down and connectivity degraded because of tectonic uplift in the gateway area.

Magnetostratigraphy and cyclostratigraphy of the Tithonian interval from the Vaca Muerta Formation, southern Neuquén Basin, Argentina

A cyclostratigraphic and paleomagnetic study was carried out in the lower part of the Vaca Muerta Formation, Los Catutos Member (Lower to lowermost Upper Tithonian) at the southern region of the Sierra de la Vaca Muerta, Neuquén Basin, west-central Argentina. The Vaca Muerta Formation is known for its unconventional resource potential and its exceptional behaviour as a world-class source rock. The studied section is c. 90 m thick and is characterized by decimetre-scale rhythmic alternations of marls, mudstones and limestones, showing a well-ordered hierarchy of cycles. Elementary cycles, bundles and superbundles were recognized with frequencies within the Milankovitch spectrum band. A total of 280 standard specimens collected from 35 paleomagnetic sampling horizons, dated with ammonites, were analyzed. The quality and primary origin of the characteristic magnetization was proved through the corresponding reversal test. A magnetostratigraphic scale was thus constructed that comprises 5 reverse and 4 normal polarity intervals, spanning the Andean Virgatosphinctes andesensis (ex mendozanus) (Lower Tithonian) to Windhauseniceras internispinosum (lowermost Upper Tithonian) ammonite Zones. Polarities were correlated with the GPTS 2016 using the equivalences between the Andean and Standard ammonite zones which, allowed to interpret that Los Catutos section was deposited between M22r to M20n Subchrons. By means of the correlation between the cyclostratigraphic and magnetostratigraphic scales, it was possible to date the Tithonian succession with unprecedented precision. Rock magnetic studies indicate that the remanence is carried by titanomagnetite.

Evidence of Unusual Geomagnetic Regimes Recorded in Plio‐Pleistocene Volcanic Sequences from the Lesser Caucasus (Southern Georgia)

AbstractWe report a detailed paleomagnetic study on two Plio‐Pleistocene lava flow sequences from the Djavakheti Highland, Lesser Caucasus. The Korkhi sequence is composed of two volcanic successions of distinct age (1.9 and 3.1 Ma), while the Apnia sequence was emplaced between 3.8 and 3.1 Ma according to available radiometric datings. Normal, reverse and intermediate polarities have been determined from both sequences. Mean directions of the normal and reverse polarity groups for each section do not match the expected field direction, but the possibility of tectonic rotations has been dismissed. A composite analysis of paleomagnetic directions, of virtual geomagnetic pole (VGP) scatter and available paleointensity results from a previous study, allow the interpretation of the observed paleomagnetic results. In the Apnia sequence, both a short recording time unable to average paleosecular variation (PSV) and an anomalous Earth magnetic field (EMF) record are responsible for the observed paleomagnetic directions. According to paleomagnetic results and radiometric ages, this sequence most probably records the reverse to normal polarity transition C2Ar to C2An‐3n. The upper Korkhi subsequence yields an anomalous EMF record, reflecting a transitional time interval. Paleomagnetic results and available absolute ages suggest that this subsequence either records transition C2r‐1r to Olduvai or Olduvai to C1r‐2r. The lower Korkhi subsequence registers a normal polarity interval within the Gauss chron, reflecting a stable stage of the EMF.

Paleomagnetic Secular Variation Constraints on the Rapid Eruption of the Emeishan Continental Flood Basalts in Southwestern China and Northern Vietnam

AbstractEstimating the duration of magma eruptions using isotopic dating methods is difficult because of the intrinsic errors of the technique regarding the dated materials (such as zircon). However, the long‐term variation of the geomagnetic field recorded by lava flows can be used to estimate the net duration of an eruption sequence. The Emeishan basalts at Dongchuan, with a thickness of 630 m, yielded a reliable characteristic remanent magnetization of normal polarity and which passed the fold test (Tauxe &amp; Watson, 1994, https://doi.org/10.1016/0012-821X(94)90006-X). Stratigraphic and magnetostratigraphic correlations of the Emeishan basalts in the Dongchuan section with other sections indicate that the eruption of the Emeishan basalts at Dongchuan spans the entire normal polarity zone in the early stage of the Emeishan large igneous province. A flow‐by‐flow analysis of geomagnetic directions of the Emeishan basalts at Dongchuan indicates that four directional groups and fifteen individual lava directions were recorded, with a net duration (excluding quiescent intervals) of no more than 3100 years. The averaged site directions from the Emeishan basalts with normal polarity conforming to a geocentric axial dipole direction indicate that this interval is longer than 104–105 years. In addition, a magnetostratigraphic study indicates that the normal polarity interval recorded by the Emeishan basalts was shorter than 2–20 × 104 years. Thus, the total duration of the normal polarity stage of the Emeishan large igneous province was roughly 105 years. There is a possible relationship between the rapid eruption and the Late Capitanian mass extinction (259.8 ± 0.4 Ma, Henderson et al., 2012).

Age and evolution of the Serbian Lake System: integrated results from Middle Miocene Lake Popovac

During the Early to Middle Miocene, a complex interplay of climate variability, sea level change, and Alpine tectonics resulted in the development of a series of long-lived lakes in the Dinarides and Serbian regions. While recent dating studies improved understanding of the Dinaride Lake System (DLS) evolution, independent age constraints are still lacking for the Serbian Lake System (SLS). Here, we present the results of an integrated study combining biostratigraphy, magnetostratigraphy, and 40Ar/39Ar radioisotopic dating of the sedimentary succession of ancient Lake Popovac to improve the chronostratigraphic framework and paleoenvironmental understanding of the SLS. Our biostratigraphic analyses of the mollusks and ostracods show endemic marker taxa for the Middle Miocene bioprovince of the SLS, such as Prososthenia fuchsi PAVLOVIC, ?Mediocypris sp., and ?Dinarocythere sp. Magnetostratigraphy revealed only one normal polarity interval, with a single reversed sample on top and, combined with an 40Ar/39ArAr weighted mean crystallization age of 14.40 ± 0.01 Ma, we correlate the studied Lake Popovac succession to Chron C5ADn, with a maximum extent from 14.61-14.16 Ma. Cyclostratigraphic analysis based on magnetic susceptibility and natural gamma radiation field logs suggest insolation forcing of the succession with an upward decrease in sedimentation rate from 25 to 12 cm/kyr. While the majority of the DLS originated during the Miocene Climatic Optimum (MCO) at ~ 17 Ma and disappeared before ~ 15 Ma, the development of the SLS started around 14.5 Ma in the Langhian. Regionally overlying Serravallian marine sediments of the Central Para - tethys imply that the Serbian Lake cycle must have ended before 13.8 Ma. Initiation of SLS deposition in the study area coincided with a peak of syn-rift extension in the Pannonian back-arc basin, which apparently also affected the Peri-Pannonian realm as far south as the study area in the Morava depression.

Magnetic stratigraphy of the Ordovician in the lower reach of the Kotuy River: the age of the Bysy-Yuryakh stratum and the rate of geomagnetic reversals on the eve of the superchron

Until recently, the existing data prevented the geophysicists from accurately dating the Bysy-Yuryakh stratum, which outcrops in the middle reach of the Kotuy River, constraining the time of its formation to a wide interval from the end of the Late Cambrian to the beginning of the Silurian. The obtained paleomagnetic data unambiguously correlate the Bysy-Yuryakh stratum to the Nyaian regional stage and constrain its formation, at least a considerable part of it, by the Tremadocian. This result perfectly agrees with the data on the Bysy-Yuryakh conodonts studied in this work and yields a spectacular example of the successful application of paleomagnetic studies in solving important tasks of stratigraphy and, correspondingly, petroleum geology. Within the Bysy-Yuryakh stratum, we revealed a large normal-polarity interval corresponding to the long (>1 Ma) period when the geomagnetic reversals were absent. This result, in combination with the data for the Tremadocian and Middle–Upper Cambrian sequences of the other regions, indicates that (1) the rate of occurrence of the geomagnetic reversals on the eve of the Ordovician Moyero superchron of reversed polarity was at most one reversal per Ma; (2) the superchron does not switch on instantaneously but is preceded by a certain gradual change in the operation conditions of the dynamo mechanism which, inter alia, manifests itself by the reduction of the frequency of geomagnetic reversals with the approach of the superchron. This finding supports the views according to which a process preparing the establishment of the superchrons takes place at the core–mantle boundary.

Middle Permian paleomagnetism of the Sydney Basin, Eastern Gondwana: Testing Pangea models and the timing of the end of the Kiaman Reverse Superchron

Paleomagnetic and geochronologic data from the eastern margin of Gondwana have been obtained from the Gerringong Volcanics in the southern Sydney Basin, Australia. The corresponding paleomagnetic pole at 56.9°S, 154.8°E (N=131; A95=9.1°) has a 40Ar/39Ar plagioclase plateau age of 265.05±0.35 [0.46] Ma from the Bumbo Latite, and overlaps with recent radio-isotopic and paleomagnetic results published from Western Gondwana. The long-documented inconsistency between Middle Permian Eastern and Western Gondwanan paleomagnetic datasets is most likely an artefact of a lack of reliable paleomagnetic data from Eastern Gondwana for this period. A number of well-dated and recently published ca. 265Ma paleomagnetic results from Gondwana and Laurussia are shown to be consistent with the Wegenerian Pangea A configuration, with a loose N-S fit of the continents for the Middle Permian. The lack of crustal overlap negates the need for a Pangea B configuration, which if valid must have been assembled to Pangea A by ca. 265Ma.The reverse polarity Bumbo Latite was sampled from the Kiaman type-section located in the southern Sydney Basin. Three cases of normal polarity were detected in the overlying Saddleback, Dapto, and Berkeley Latites, previously assigned to the Kiaman Reverse Superchron (KRS). We review KRS-aged magnetostratigraphic data and propose that an age assignment of 265Ma most likely represents the termination of the non-reversing field, with longer stable intervals of normal polarity recorded and able to be correlated globally.

High northern geomagnetic field behavior and new constraints on the Gilsá event: Paleomagnetic and 40Ar/39Ar results of ∼0.5–3.1 Ma basalts from Jökuldalur, Iceland

Recent paleomagnetic results of extrusive rocks from high southern latitudes (>60°S) and high northern latitudes (>60°N) have been suggested to reflect a hemispheric asymmetry of the geomagnetic field on time-scales of 105 to 106 yrs, with higher and more stable fields in the north. This interpretation, however, is based on only a few modern-standard paleodirectional data sets and on high northern stable field paleointensity data of rocks that are mainly younger than 100 kyr. The sparsity of modern-standard data questions the validity (and age range) of this potential geomagnetic asymmetry. In 2013 and 2014, we sampled basaltic lava flows in Jökuldalur, north-eastern Iceland, to obtain high-standard paleodirectional and paleointensity data at relatively high-northern latitudes (65.2°N). On average, we sampled >15 cores per site at 51 sites of predominantly Matuyama age. Complete demagnetization was carried out on all samples using AF or thermal demagnetization. We present 45 distinct paleomagnetic directions based on overall N>10 ChRMs per site and α95<3.5°. We obtain a mean direction of D=355.7°, I=76.3°, and α95=3.2 for N=45 sites that is not significantly different from a GAD field. The resulting 45 VGPs distribute around the North Pole, and the global mean paleomagnetic pole (λ¯=87.8°, ϕ¯=224.3°) is coincident with the North Pole within the α95 confidence limit. We calculate a VGP dispersion SB(Mat)=20.523.317.8 and an average inclination anomaly ΔI=−0.91°2.94−4.88 for our 38 Matuyama age data. The dispersion SB overall supports the interpretation of a dependence of SB on latitude during the Matuyama, while the negligible ΔI suggests little deviation from a GAD field. Based on relatively strict cut-off criteria we also present six new field strength estimates from the time interval ∼1.2–1.83 Ma, thus filling a large data gap of the high-northern stable field behavior. We obtain a median VADM of 57±3ZAm2 (VDM of 60±5Am2), which is higher than the median VADM of 16 intensity estimates from Antarctica (39±7 ZAm2) from the same period. A higher northern field is also found when using less strict cut-off criteria resulting in 14 field estimates from Jökuldalur, i.e. we find support for higher field strength in the northern hemisphere as compared to the southern hemisphere during the Matuyama. Finally, we deliver a revised magneto-chronostratigraphic model of Jökuldalur and conduct an investigation of the type sections of the so-called Gilsá normal polarity event around 1.62 Ma. Our revised model is based on 11 new 40Ar/39Ar ages. No evidence is found of the existence of the Gilsá event in Jökuldalur. Instead we find that the normal polarity intervals in the type sections can both be correlated to Olduvai subchron.

A biochronologic tie-point for the base of the Tortonian stage in European terrestrial settings: Magnetostratigraphy of the topmost Upper Freshwater Molasse sediments of the North Alpine Foreland Basin in Bavaria (Germany)

Chronostratigraphic correlation and dating of terrestrial, especially mammal bearing, sediments of the European Neogene are still problematic and highly debated. In particular, absolute ages for important vertebrate assemblages are often not available making correlation during the continental Miocene across Europe so ambiguous. Here we present a detailed magnetostratigraphic study on a paleontological key section of the Middle to Late Miocene transition in the North Alpine Foreland Basin (NAFB) in Bavaria (Hammerschmiede) and a neighboring drill core, which has a total length of 150.4 m and includes, stratigraphically, the fossil site. We identify three complete normal polarity intervals, which have been correlated based on biochronologic constraints to chrons C5An.1n, C5r.2n and C5r.2r-1n. At least two major hiatuses probably occurred within the interjacent reversed chrons, for which geological indications are present and might be relatable to isochronic features in the Vienna basin. Inferred upper limits of the accumulation rate vary between 10 and 30 cm/kyr. This correlation determines the age of the Hammerschmiede vertebrate level HAM 5 to be about 11.62 Ma, making it an ideal biochronologic tie-point for the base of the Tortonian and Pannonian stages in terrestrial settings. Additionally, we date the youngest freshwater molasse lithostratigraphic unit of the Bavarian part of the NAFB, the Obere Serie, to between 13.8 and 11.1 Ma. Following our correlation, the lack of hipparion horses in the Bavarian part of the NAFB has stratigraphic rather than ecologic reasons and the Ilipparion datum' seems to be a single bio-event at 11.1 Ma in Western Eurasia.