Magnetostratigraphy

Dating the PTB in in the red beds of Russia

There is a major problem in studying the Permian-Triassic mass extinction, and that is to correlate the marine and terrestrial rocks. After the Permian-Triassic boundary (PTB) was officially established in 2001 at Meishan in China (Yin et al. 2001), the question was how to match this to rocks deposited in rivers and lakes.

Until recently, the stratigraphic system for Middle and Late Permian continental beds was based largely on vertebrate biozones stemming from the Karoo successions in South Africa (Rubidge 1995). The famous tetrapod zones of the Karoo (see Section left), the Dicynodon Assemblage Zone, the Lystrosaurus Assemblage Zone, the Cynognathus Assemblage Zone, and so on were founded on their commonest vertebrate fossils, and so could be recognized readily by field collectors. However, there was minimal evidence for linking these tetrapod biozones to the global marine standard

New radiometric dates from different levels in the Karoo succession provide additional evidence to correlate the terrestrial and marine successions (Rubidge et al. 2013), but such dating is not always possible. In the case of the northern palaeohemisphere sections in Russia (Tverdokhlebov et al. 2003, 2005; Benton 2015), other methods are required.

It was long assumed that the Russian Tatarian was equivalent to the Late Permian (e.g. Efremov 1937; Olson 1957; Chudinov 1965; Benton et al. 2004). However, we had the shock of our lives back in 2004 when we opened the latest edition of the Cambridge timescale project (Gradstein et al. 2004), and found that the Tatarian and Kazanian had been moved back and dated as Middle Permian rather than Late Permian. If correct, this would have implied a 9-10 myr gap in the Russian stratigraphic record (as well as in the South African succession, correlated roughly by shared tetrapod genera). It would have meant that the palaeontologists who thought they had been documenting tetrapod evolution through the PTB in Russia and South Africa would turn out to have been deluded!

Fortunately, the error was rectified in the following versions of the time scale (Ogg et al. 2008; Gradstein et al. 2012), in which the old Russian ‘Upper Permian’ (Ufimian, Kazanian, Tatarian) has been stretched to occupy most of the Middle Permian and all of the Upper Permian.

Magnetostratigraphy, in the field

Magnetostratigraphy is an alternative to these approaches as it utilises the globally synchronous nature of magnetic reversals and is, essentially, a facies-independent technique. It does however rely upon the construction of a coherent and composite magnetostratigraphic record, linked to biostratigraphy, and significant progress has been made toward a global, composite Permian-Triassic Boundary (PTB) record (Scholger et al. 2000; Molostovskii 2005; Steiner 2006; Szurlies 2007; Hounslow and Balabanov 2018).

The magnetostratigraphy of Permian-Triassic sections in Russia has a long history of study commencing in the late 1950s and includes the work of Khramov, Molostovskii, Borisov, and Burov and their colleagues. However, much of the work was published in Russia, and often in regional conference volumes, and remains difficult to access; furthermore there were significant concerns about the demagnetisation techniques employed (Bazhenov 2008). The most comprehensive and recent information (in English) is that for the Volga and Kama areas (Burov et al. 1998) some 600-700 km N-NW of our study area. In addition to a composite and individual magnetostratigraphic sections, this summary of the available information also presents basic data for each section, which helps to assess the reliability of the information.

[Left] Graeme Taylor adopts a heroic pose, as he drills into a bed of red sandstone, trying to aim the hollow drill parallel to bedding. Mike Benton acts as field assistant, carrying a large plastic bottle of water which is pumped over the hollow drill pit continuously to prevent damage.

[Right] Graeme measures the current orientation of the core by inserting his directional measuring tool into the hole from which the core has been removed. The present orientation can be converted into a Late Permian orientation.

We collected numerous samples of red-bed sandstones during the 2006 Expedition, and these were analysed for their original magnetization, whether ‘normal’ or ‘reversed’, to check their location in magnetostratigraphic scales, and also to determine the orientation of the Late Permian north pole, and so to check palaeogeographic reconstructions.

[Left] Collection of cores taken from different sandstones, catalogued, numbered, and marked with a way-up. These were taken back to the geomagnetism lab in Plymouth, where their north/ south normal/ reversed magnetizations were measured.

Our study (Taylor et al. 2009) of the Permian-Triassic boundary of the southern Cis-Urals aims to (a) resolve the issue of whether or not there is a major temporal gap below the PTB in this part of Russia, and (b) contribute to the global magnetostratigraphic record of this crucial interval in Earth history. This study therefore has focussed upon sampling across the supposed PTB, concentrating in particular on the uppermost Tatarian deposits of the Vyatskian Gorizont immediately below the locally recognised PTB.

Our results

Our paper (Taylor et al. 2009) presented the evidence that there was no long hiatus spanning the Upper Permian in Russia, nor in other areas correlated with the Russian Ufimian, Kazanian, and Tatarian zones. We concluded:

The palaeomagnetic data yield a distinct series of polarity zones that provide clear local and regional correlation and are readily tied to a recently compiled global magnetostratigraphic record. On the basis of this correlation the sampled sections span the upper Guadalupian to Induan stages without any obvious break, so confirming the traditional view that the Tatarian is Late Permian in age. Anomalies in the magnetic inclination are consistent with sediment compaction (inclination shallowing, a common phenomenon of red beds) but declination anomalies between these sites and elsewhere in Russia may suggest localised vertical axis rotation.

The most complete section sampled was at Boyevoya Gora, spanning Upper Permian and Lower Triassic, and this was used as the reference with which to compare the other sections. With the exception of measurements from Tuyembetka, the locally identified PTB (defined on facies and fossil evidence) falls close to a polarity transition from R to N polarities, but always within the lower part of the N interval. The PTB falls in the lower part of this N polarity chron and beneath a short R event within the N chron (Steiner, 2006). This short-lived R event (t1 of figure left) is present in the Boyevaya Gora, Tuyembetka and Krasnogor sections and may also be present in the other two sections where reversed polarity was detected in isolated samples at or near the top of the sampled sections.

[Left] Correlation of the studied sections and a composite section for the Orenburg region.

References

  • Bazhenov, M.L. A.N. Grishanov, R.V.d. Voo, N.M. Levashova. 2008. Late Permian palaeomagnetic data east and west of the Urals. Geophysical Journal International 173, 395-408.
  • Benton, M.J. 2015. When life nearly died: the greatest mass extinction of all time, 2nd edition Thames & Hudson, London.
  • Benton, M.J., Tverdokhlebov, V.P. and Surkov, M.V. 2004. Ecosystem remodelling among vertebrates at the Permian-Triassic boundary in Russia. Nature 432, 97-100. pdf. Download the original of Figure 1 as a gif or pdf. Download the Excel data file here.
  • Burov, B.V., I.Y. Zharkov, D.K. Nurgaliev, Y.P. Balabanov, A.S. Borisov, P.G. Yasonov. 1998. Paleomagnetic characteristics of Upper Permian sediments – Magnetostratigraphic characteristics of Upper Permian Sections sections in the Volga and the Kama areas in: N.K. Esaulova, V.R. Lozovskii, A.Y. Rozanov, (eds), Stratotypes and reference sections of the Upper Permian in the regions of the Volga and Kama rivers. Nauka, Moscow, 300 pp..
  • Chudinov, P.K. 1965. New facts about the fauna of the Upper Permian of the USSR. Journal of Geology 73, 117-130.
  • Efremov, I.A. 1937. [On the stratigraphic divisions of the continental Permian and Triassic of the USSR, based on tetrapod faunas.] Doklady AN SSSR 16, 125-132.
  • Gradstein, F.M., J.G. Ogg, A.G. Smith (eds). 2004. A Geologic Time Scale 2004. Cambridge University Press, Cambridge.
  • Gradstein, F.M., Ogg, J.G., Schmitz, M.D., and Ogg, G.M. (eds). 2012. The Geological Time Scale 2012. Elsevier, Amsterdam, 2 vols., 1144 p.
  • Hounslow, M. and Balabanov, Y.P. 2018. A geomagnetic polarity timescale for the Permian, calibrated to stage boundaries. In: Lucas, S.G. & Shen, S.Z. (eds) The Permian Timescale. Geological Society, London, Special Publications, 450.
  • Molostovskii, E.A. 2005. Magnetostratigraphic correlation of Upper Permian marine and continental formations, stratigraphy and geological correlation. Stratigrafiya Geologicheskaya Korrelyatsiya, 13, 49-58.
  • Ogg, J.G., Ogg, G., and Gradstein, F.M. 2008. The concise geologic time scale Cambridge University Press, Cambridge.
  • Olson, E.C. 1957. Catalogue of localities of Permian and Triassic vertebrates of the territories of the U.S.S.R. Journal of Geology 65, 196-226.
  • Rubidge, B. S. (ed.) 1995. Biostratigraphy of the Beaufort Group (Karoo Supergroup), South Africa. Council of Geoscience, Pretoria, 72 pp.
  • Rubidge, B., Erwin, D.H., Ramezani, J., Bowring, S.A, and de Klerk, W.J. 2013. High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U–Pb zircon constraints from the Karoo Supergroup, South Africa. Geology 41, 363-366
  • Scholger, R., H.J. Mauritsch, R. Brandner. 2000. Permian-Triassic boundary magnetostratigraphy from the Southern Alps (Italy) Earth and Planetary Science Letters, 176, 495-508.
  • Steiner, M.B. 2006. The magnetic polarity time scale across the Permian-Triassic boundary, in: S.G. Lucas, G. Cassinis, J.W. Schneider (eds), Non-marine Permian biostratigraphy and biochronology. Geological Society, London, Special Publications 265, 15-38.
  • Szurlies, M. 2007. Latest Permian to Middle Triassic cyclo-magnetostratigraphy from the Central European Basin, Germany: Implications for the geomagnetic polarity timescale. Earth and Planetary Science Letters, 261, 602-619.
  • Taylor, G.K., Tucker, C., Twitchett, R.J., Kearsey, T., Benton, M.J., Newell, A.J., Surkov, M.V., and Tverdokhlebov, V. P. 2009. Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus. Earth & Planetary Science Letters 281, 36-47. pdf.
  • Tverdokhlebov, V.P., Tverdokhlebova, G.I., Minikh, A.V., Surkov, M.V., and Benton, M.J. 2005. Upper Permian vertebrates and their sedimentological context in the South Urals, Russia. Earth-Science Reviews, 69, 27-77. pdf
  • Tverdokhlebov, V.P., Tverdokhlebova, G.I., Surkov, M.V., and Benton, M.J. 2003. Tetrapod localities from the Triassic of the SE of European Russia. Earth-Science Reviews 60, 1-66. pdf
  • Yin, H., Zhang, K., Tong, J., Yang, Z. and Wu, S. 2001. The Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary. Episodes 24, 102-114.