Characterization of Lower and Middle Pleistocene tephra beds in the southern plains of western Canada

Wellsch Valley tephra, near Swift Current, southwestern Saskatchewan, and Galt Island tephra, near Medicine Hat, southeastern Alberta, have been referenced in the literature since the 1970s, but little is available on their physical and chemical attributes – necessary information if they are to be recognized elsewhere. This study seeks to remedy this situation. Both have a calc-alkaline rhyolitic composition with hornblende, biotite, plagioclase, pyroxene, and Fe-Ti oxides being dominant. They have a similar composition but are not the same. Wellsch Valley tephra has a glass fission-track age of 0.75 ± 0.05 Ma, a reversed magnetic polarity, and was deposited at the close of the Matuyama Chron. Galt Island tephra has an age of 0.49 ± 0.05 Ma, a normal magnetic polarity, and was deposited during the early Brunhes Chron. Rich fossil vertebrate faunas occur in sediments close to them. Major- and trace-element concentrations in their glass shards indicate a source in the Cascade Range of the Pacific Northwest, USA, but differences in trace-element ratios suggest they are not consanguineous.

The Carboniferous timescale: an introduction

The Carboniferous chronostratigraphic scale consists of two subsystems, six series and seven stages. Precise numerical age control within the Carboniferous is uneven, and a global magnetic polarity timescale for the Carboniferous is far from established. Isotope stratigraphy based on Sr, C and O isotopes is in an early stage but has already identified a few Sr and C isotope events of use to global correlation. Cyclostratigraphy has created a workable astrochronology for part of Pennsylvanian time that needs better calibration. Chronostratigraphic definitions of most of the seven Carboniferous stages remain unfinished. Future research on the Carboniferous timescale should focus on GSSP selection for the remaining, undefined stage bases, definition and characterization of substages, and further development and integration of the Carboniferous chronostratigraphic scale with radioisotopic, magnetostratigraphic, chemostratigraphic and cyclostratigraphic tools for calibration and correlation and the cross correlation of nonmarine and marine chronologies.

Cosmic-Ray Transport in Heliospheric Magnetic Structures. III. Implications of Solar Magnetograms for the Drifts of Cosmic Rays

The transport of energetic particles in the heliosphere is reviewed regarding the treatment of their drifts over an entire solar cycle including the periods around solar maximum, when the tilt angles of the heliospheric current sheet increase to large values and the sign of the magnetic polarity changes. While gradient and curvature drifts are well-established elements of the propagation of cosmic rays in the heliospheric magnetic field, their perturbation by the solar-activity-induced large-scale distortions of dipole-like field configurations and by magnetic turbulence is an open problem. Various empirical or phenomenological approaches have been suggested, but either lack a theory-based motivation or have been shown to be incompatible with measurements. We propose a new approach of more closely investigating solar magnetograms obtained from GONG maps, leading to a new definition of (i) tilt angles that may exceed those provided by the Wilcox Solar Observatory during high activity and of (ii) a “noninteger sign” that can be used to reduce the drifts during these periods as well as to provide a refinement of the magnetic field polarity. The change of sign from A < 0 to A > 0 of solar cycle 24 can be in this way localized to occur between Carrington Rotations 2139 and 2140 in mid 2013. This treatment is fully consistent in the sense that the transport modeling uses the same input data to formulate the boundary conditions at the heliobase as do the magnetohydrodynamic models of the solar wind and the embedded heliospheric magnetic field that exploit solar magnetograms as inner boundary conditions.

Age constraints for the Trachilos footprints from Crete

We present an updated time frame for the 30 m thick late Miocene sedimentary Trachilos section from the island of Crete that contains the potentially oldest hominin footprints. The section is characterized by normal magnetic polarity. New and published foraminifera biostratigraphy results suggest an age of the section within the Mediterranean biozone MMi13d, younger than ~ 6.4 Ma. Calcareous nannoplankton data from sediments exposed near Trachilos and belonging to the same sub-basin indicate deposition during calcareous nannofossil biozone CN9bB, between 6.023 and 6.727 Ma. By integrating the magneto- and biostratigraphic data we correlate the Trachilos section with normal polarity Chron C3An.1n, between 6.272 and 6.023 Ma. Using cyclostratigraphic data based on magnetic susceptibility, we constrain the Trachilos footprints age at ~ 6.05 Ma, roughly 0.35 Ma older than previously thought. Some uncertainty remains related to an inaccessible interval of ~ 8 m section and the possibility that the normal polarity might represent the slightly older Chron C3An.2n. Sediment accumulation rate and biostratigraphic arguments, however, stand against these points and favor a deposition during Chron C3An.1n.

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

We report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a strati­graphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagneti­zation. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The sta­tistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmerid­gian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is esti­mated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (&gt;8 cm/k.y.) and two periods of low (&lt; 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ~44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Magnetic Signature of the Saco do Mamanguá and Paraty-Mirim Cove, Rio de Janeiro, Brazil

ABSTRACT. In this work, we investigate the N40°-50°E-oriented magnetic lineaments observed in the marine and aeromagnetic data from the Paraty-Mirim and Saco do Mamanguá (RJ) coves. The main characteristic of these lineaments is the presence of normal and reverse magnetic polarity, indicating that the bodies responsible for the anomalies may be of different ages or have suffered remagnetization effects by metamorphism or have distinct lithologies. The interpretative model of the anomalies was obtained by the 3D Magnetization Vector Inversion (MVI) and by the study of the geology and tectonics of the area. Our results suggest, as a more likely hypothesis, that the magnetic lineaments can be correlated with two tholeiitic dykes swarms, Resende-Ilha Grande and Serra do Mar, which have intruded the region at different times. The age formation of those swarms can be placed at time intervals where the Earth's magnetic field has normal and reverse polarities. The contrasts of magnetic susceptibility obtained from the models are consistent with contrasts of the lithology of basic dykes intruded in the gneissic-granitic basement of the region. Keywords: magnetization vector inversion, magnetic properties, magnetic lineaments, remanent magnetization, tholeiitic dykes swarms Assinatura Magnética do Saco do Mamanguá e da Enseada de Paraty-Mirim, Rio de Janeiro, BrasilRESUMO. Neste trabalho, investigamos os lineamentos magnéticos orientados a N40°-50°E observados nos dados marinhos e aeromagnéticos das enseadas de Paraty-Mirim e do Saco do Mamanguá (RJ). A principal característica destes lineamentos é a presença de polaridade magnética normal e reversa, indicando que os corpos responsáveis pelas anomalias podem ter idades diferentes ou terem sofrido efeitos de remagnetização por metamorfismo ou apresentarem litologias distintas. O modelo interpretativo das anomalias foi obtido pela Inversão do Vetor de Magnetização (IVM) 3D e pelo estudo da geologia e tectônica da área. Nossos resultados sugerem, como hipótese mais provável, que os lineamentos magnéticos podem ser correlacionados com dois enxames de diques toleíticos, Resende-Ilha Grande e Serra do Mar, que têm idades diferentes de intrusão. As idades de formação desses enxames podem ser colocadas em intervalos de tempo em que o campo magnético da Terra apresentou polaridades normal e reversa. Os contrastes de susceptibilidade magnética obtidos no modelo são coerentes com os contrastes de litologia de diques básicos intrudidos no embasamento granítico-gnáissico da região.Palavras-chave: inversão do vetor de magnetização, propriedades magnéticas, lineamentos magnéticos, magnetização remanente, enxames de diques toleíticos

A tale of two Tweefonteins: What physical correlation, geochronology, magnetic polarity stratigraphy, and palynology reveal about the end-Permian terrestrial extinction paradigm in South Africa

The contact between the Daptocephalus to Lystrosaurus declivis (previously Lystrosaurus) Assemblage Zones (AZs) described from continental deposits of the Karoo Basin was commonly interpreted to represent an extinction crisis associated with the end-Permian mass-extinction event at ca. 251.901 ± 0.024 Ma. This terrestrial extinction model is based on several sections in the Eastern Cape and Free State Provinces of South Africa. Here, new stratigraphic and paleontologic data are presented for the Eastern Cape Province, in geochronologic and magnetostratigraphic context, wherein lithologic and biologic changes are assessed over a physically correlated stratigraphy exceeding 4.5 km in distance. Spatial variation in lithofacies demonstrates the gradational nature of lithostratigraphic boundaries and depositional trends. This pattern is mimicked by the distribution of vertebrates assigned to the Daptocephalus and L. declivis AZs where diagnostic taxa of each co-occur as lateral equivalents in landscapes dominated by a Glossopteris flora. High-precision U-Pb zircon (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) age results indicate maximum Changhsingian depositional dates that can be used as approximate tie points in our stratigraphic framework, which is supported by a magnetic polarity stratigraphy. The coeval nature of diagnostic pre- and post-extinction vertebrate taxa demonstrates that the L. declivis AZ did not replace the Daptocephalus AZ stratigraphically, that a biotic crisis and turnover likely is absent, and a reevaluation is required for the utilization of these biozones here and globally. Based on our data set, we propose a multidisciplinary approach to correlate the classic Upper Permian localities of the Eastern Cape Province with the Free State Province localities, which demonstrates their time-transgressive nature.