A geomagnetic polarity timescale for the Carboniferous

The geomagnetic polarity pattern for the Carboniferous is incompletely known, but with the best resolved parts in the Serpukhovian and Bashkirian. Hence, data from both igneous and sedimentary units are also used in an additional polarity bias evaluation. In the Tournaisian to mid Visean interval polarity is mainly derived from palaeopole-type palaeomagnetic studies, allowing identification of polarity bias chrons. Seven polarity bias chrons exist in the Mississippian (MI1nB to MI4nB) with an additional 33 conventional magnetochrons and submagnetochrons (MI4r to MI9r). The Moscovian and Gzhelian polarity is best resolved in magnetostratigraphic studies from the Donets Basin and the southern Urals. Dispute about the reliability of these data is ill-founded, since an assessment of supporting data from palaeopole-type studies suggests that these datasets currently provide the best magnetic polarity data through the Pennsylvanian. Polarity bias assessment indicates a normal polarity bias zone in the Kasimovian. In the Pennsylvanian there are 27 conventional magnetochrons and submagnetochrons (PE1n to CI1r) and one normal polarity bias chron (PE8nB). The Kiaman Superchron begins in the mid Bashkirian, with clear data indicating brief normal polarity submagnetochrons within the Superchron. The magnetochron timescale is calibrated using 31 U-Pb zircon dates and a quantitative Bayesian-based age-scaling procedure

Septarian concretions

Septarian structures are former cracks, often filled with cement and are most commonly found in concretions hosted in mudrocks, although rare occurrences are known from siltstones and sandstones. Septarian structures occur in concretions or concretionary sheets, which may be chemically and mineralogically the same as non-septarian concretions in the same mudrock. The host concretions are most commonly calcite, dolomite or siderite dominated, although rare occurrences have been interpreted from silica concretions. Septarian concretions are predominantly a pre-Pleistocene phenomenon, although a potential “proto” version has been described from the late Pleistocene (Duck, 1995). Crack morphology Septarian structures were initially formed as open fractures, and are most often concentrated in the central regions of concretions and reduce in width and frequency toward the outer parts of the concretion, which may or may not be cracked. The fractures take a variety of forms, from lenticular..

Magnetostratigraphy of the Lower Triassic beds from Chaohu(China) and its implications for the Induan–Olenekian stage boundary.

A magnetostratigraphic study was performed on the lower 44 m of the West Pingdingshan section near Chaohu city, (Anhui province, China) in order to provide a magnetic polarity scale for the early Triassic. Data from 295 paleomagnetic samples is integrated with a detailed biostratigraphy and lithostratigraphy. The tilt-corrected mean direction from the West Pingdingshan section, passes the reversal and fold tests. The overall mean direction after tilt correction is D=299.9º, I=18.3º (κ=305.2, α95=1.9, N=19). The inferred paleolatitude of the sampling sites (31.6ºN, 117.8ºE) is about 9.4º, consistent with the stable South China block (SCB), though the declinations indicate some 101o counter-clockwise rotations with respect to the stable SCB since the Early Triassic. Low-field anisotropy of magnetic susceptibility indicates evidence of weak strain. The lower part of the Yinkeng Formation is dominated by reversed polarity, with four normal polarity magnetozones (WP2n to WP5n), with evidence of some thinner (<0.5 m thick) normal magnetozones. The continuous magnetostratigraphy from the Yinkeng Formation, provides additional high-resolution details of the polarity pattern through the later parts of the Induan into the lowest Olenekian. The magnetostratigraphic and biostratigraphic data shows the conodont marker for the base of the Olenekian (first presence of Neospathodus waageni) is shortly prior to the base of normal magnetozone WP5n. This provides a secondary marker for mapping the base of the Olenekian into successions without conodonts. This section provides the only well-integrated study from a Tethyan section across this boundary, but problems remain in definitively relating this boundary into Boreal sections with magnetostratigraphy

Intercalibration of Boreal and Tethyan timescales: the magneto-biostratigraphy of the Middle Triassic and the latest Early Triassic from Spitsbergen, Arctic Norway

An integrated bio-magnetostratigraphic study of the latest Early Triassic to the upper parts of the Middle Triassic, at Milne Edwardsfjellet in central Spitsbergen, Svalbard, allows a detailed correlation of Boreal and Tethyan biostratigraphies. The biostratigraphy consists of ammonoid and palynomorph zonations, supported by conodonts, through some 234 m of succession in two adjacent sections. The magnetostratigraphy consists of ten substantive normal–reverse polarity chrons defined by sampling at 150 stratigraphic levels. The magnetization is carried by magnetite and an unidentified magnetic sulphide, and is difficult to fully separate from a strong present-day like magnetization. The bio-magnetostratigraphy from the late Olenekian (Vendomdalen Member) is supplemented by data from nearby Vikinghøgda. The early and mid-Anisian has a high sedimentation rate, comprising over half the ca. 140-m thickness of the Botneheia Formation, whereas the late Anisian and lower Ladinian is condensed into about 20 m. The two latest Boreal Ladinian ammonoid zones are absent due to erosional truncation below the Tschermakfjellet Formation. Correlation to Tethyan bio-magnetostratigraphies shows the traditional base of the Boreal Anisian (base of G. taimyrensis Zone) precedes the base Anisian (using here definitions based on the Desli Caira section in Romania). The Boreal upper Anisian G. rotelliforme and F. nevadanus ammonoid zones correlate to most of the Tethyan Pelsonian and Illyrian substages. The base Ladinian defined in the Tethyan global boundary stratotype and point (GSSP) is closely equivalent to the traditional base of the Boreal Ladinian at the I. oleshkoi Zone. The latest Olenekian to early Anisian magnetic polarity timescale is refined using the Spitsbergen data

Albertiana Working Group Report:The case for the Global Stratotype Section and Point (GSSP) for the base of the Norian stage

The Norian Stage is the longest stage in the Phanerozoic, and consequently it is important to define its base precisely for chronostratigraphy with a global stratotype section and point (GSSP). Previous work over many decades has indicated two possible candidate sections at Black Bear Ridge (British Columbia, Canada) and Pizzo Mondello (Sicily, Italy). Based on prior datasets the working group evaluated the global correlation potential of the two proposed primary markers, the conodont Metapolygnathus parvus and ‘flat-clam’ Halobia austriaca. We also evaluated secondary markers for these boundary datums for correlation, and the veracity of the proposed sections for GSSP status. The factual data and arguments for the proposed sections and datums are presented here. Through a two-stage process of option elimination in a voting process conforming with ICS guidelines, the working group decided by 60% majority to propose that the first occurrence datum (FAD) of Halobia austriaca in the Pizzo Mondello section at the base of bed FNP135A should become the ‘golden spike’ for the Norian. A secondary biotic marker for this boundary is the FAD of Primatella gulloae, in sample NA43, ca. 0 m below FNP135A and the FAD of Dimorphites noricus (sample NA42.1) ca. 3.5 m above bed FNP135 (indicating first subzone of the Jandianus Zone). The best physical secondary marker is magnetozone PM5n with the proposed boundary ca.40% from the base of PM5n. The strengths of this choice are that it also mains historical priority for ammonoid zonations, which placed the base Norian near to this level in Europe, North America and probably NE Asia, and Halobia austriaca is widely distributed in all paleolatitudes and is well established taxon

Rotational remanent magnetisation as a magnetic mineral diagnostic tool at low rotation rates

Summary Prior work on rotational remanent magnetisation (RRM) and rotational anhysteretic remanent magnetisation (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of datasets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25 to 3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilised (at 100 mT AF field, 100μT bias field). Sets of magnetic mineral extracts from sediments, and well characterised rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM- ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values, with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain-size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of -83 to -109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0 -1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterisation of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetisation imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation

Foraminifers in the Holkerian Stratotype, regional substage in Britain:key taxa for the Viséan subdivision

Foraminiferal revision of the Holkerian Stratotype of Britain at Barker Scar, Holker Hall, south Cumbria, UK, allows the subdivision of the section into the Cf4δ, Cf5α and Cf5β subzones (the latter being further subdivided into lower Cf5β1 and upper Cf5β2 intervals). The base of Cf5α subzone at the base of bed C and base of Cf5β subzone from the middle part of bed C, occur at 14 m and 10.5 m, respectively, below the traditional basal boundary of the Holkerian at bed K. The lower boundaries of these foraminiferal subzones occur within the main interval affected by dolomitization in the section, which poses problems in defining precisely the bases for these subzones. Nevertheless, in spite of the dolomitization, a more or less continuous foraminiferal record allows a solid correlation of the base of the Cf5β with the preserved succession in the Livian Substage (defined in Belgium, but also used in France), and it is assumed that the base of this substage should correspond to the base of the Cf5α subzone. The base of the Cf5α subzone can be correlated with the base of the Russian Tulian Substage, since it contains many taxa in common with the Holkerian. However, further investigation is needed to establish other levels of correlation (e. g., base of the Cf5β subzone) higher up in the Holkerian substage. All of these problems suggest that the Holkerian, as it is currently recognised, and the Barker Scar stratotype section, in particular, should be reconsidered, and a new para-stratotype section, ideally devoid of dolomitization, should be located and investigated, in order to corroborate the occurrence of the Cf5α and Cf5β foraminiferal subzones compared to those recognised in the Barker Scar Stratotype. These modifications would allow identification of an apparent synchronous faunal event forming the basis of a future subdivision of the Viséan

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

SUMMARY Prior work on rotational remanent magnetization (RRM) and rotational anhysteretic remanent magnetization (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of data sets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25–3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilized (at 100 mT AF field, 100 μT bias field). Sets of magnetic mineral extracts from sediments, and well characterized rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM-ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values (up to 6 μT), with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of –83 to –109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0–1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterization of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetization imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.</jats:p

Episodic zircon age distributions mimic fluctuations in subduction

Decades of geochronological work have shown the temporal distribution of zircon ages to be episodic on billion-year timescales and seemingly coincident with the lifecycle of supercontinents, but the physical processes behind this episodicity remain contentious. The dominant, end-member models of fluctuating magmatic productivity versus selective preservation of zircon during times of continental assembly have important and very different implications for long-term, global-scale phenomena, including the history of crustal growth, the initiation and evolution of plate tectonics, and the tempo of mantle outgassing over billions of years. Consideration of this episodicity has largely focused on the Precambrian, but here we analyze a large collection of Phanerozoic zircon ages in the context of global, full-plate tectonic models that extend back to the mid-Paleozoic. We scrutinize two long-lived and relatively simple active margins, and show that along both, a relationship between the regional subduction flux and zircon age distribution is evident. In both cases, zircon age peaks correspond to intervals of high subduction flux with a ~10-30 Ma time lag (zircons trailing subduction), illuminating a possibly intrinsic delay in the subduction-related magmatic system. We also show that subduction fluxes provide a stronger correlation to zircon age distributions than subduction lengths do, implying that convergence rates play a significant role in regulating the volume of melting in subduction-related magmatic systems, and thus crustal growth

High-resolution definition and correlation of the Asbian-Brigantian boundary in northern England and the Scottish borders, using foraminiferal diversity and richness

Foraminiferal diversity and taxa richness from beds transitional between the Asbian and Brigantian substages (Middle Mississippian) show patterns of secular change which allow detailed inter-regional correlations to be established. Foraminifera from the Askrigg Block, Stainmore Trough, Alston Block, South Cumbria Shelf and Solway Basin show similar secular changes (foraminiferal trends, FTs), allowing correlation to be made with the basal Brigantian Stratotype at Janny Wood. Despite the absence of consistent microfossil first occurrence markers for the recognition of the base of the Brigantian, this horizon can be confidently recognised by means of foraminiferal trends. The FTs allow the precise location of the base of the correlated Brigantian in sections where this boundary was questioned or controversial in previous studies, as well as to amend the position of the foraminiferal zones and subzones during the late Asbian and basal Brigantian. This type of analysis when used in combination with foraminiferal zonations, emergent surfaces and lithological cyclicity, together, provide a robust means for high-resolution correlation. This methodology, provides the least uncertainty in sections that have been most densely sampled, whereas for less intensely sampled sections there is more correlation uncertainty