Ancient platform carbonates with well-developed Lofer cyclicity: New candidates for magnetostratigraphy and geodynamically oriented palaeomagnetism

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Abstract

Young platform carbonates seem to preserve continuous records of the magnetic field. In contrast, ancient ones often fail to yield a meaningful palaeomagnetic signal. The frequent failure of the palaeomagnetic method in the platform carbonates of the Alpine-Mediterranean zone resulted in an investigation as to what extent the original lithology is responsible for this situation. The material selected for the study represents Late Triassic platform facies with well-developed Lofer cycles (metre-scale peritidal-subtidal cycles) of the Transdanubian Central Range in Hungary, with evidence for minimal late diagenetic, tectonic or secondary alteration processes. By selecting an area where the expected declinations for the Late Triassic are rotated in a counterclockwise manner by a significant amount, relative to the present north, we hope to recognize ancient natural remanence on the basis of both consistency and departure from the Earth's present field direction. For the study, different members of Lofer cycles were drilled at two sections (13 km apart). 71 and 38 samples, respectively, were taken from the two sections and fully oriented in the field. Using standard palaeomagnetic processing techniques, ancient natural remanence was isolated in all 'A', 'B' and 'C' members of the Lofer cycle, although different members were characterized by different magnetic parameters ('C' members were diamagnetic with very low NRM intensity; 'A' members had positive susceptibility and relatively strong NRM, and 'B' members intermediate properties). The mean palaeomagnetic directions isolated for the two sections, based on 58 and 30 samples, respectively, are D=306°, I=47°, k=43, α95=3° and D=300°, I=52°, k= 26, α95=5° before and Dc=313°, Ic=52 and Dc=307°, Ic=52° after tectonic correction. Both normal and reversed polarities were observed, and there were several polarity reversals, even in short sections. These results suggest that platform carbonates with Lofer cycles are likely to preserve primary remanence, thus they are suitable for magnetostratigraphy studies, if free from karstic cavities and fissures. The presence of 'A' members is very favourable for the preservation of the original magnetization (since general water circulation is inhibited by these members), but these may represent condensed intervals, thus the sedimentary record in these members is not necessarily complete. Despite the above limitations and also the possibility of gaps at sequence boundaries, platform carbonates are good candidates for future definition of magnetostratigraphic scales, for example in the Late Triassic; they also promise palaeomagnetic results of considerable tectonic importance, since platform carbonates with Lofer cycles are widespread in the Alpine-Mediterranean zone.

Original languageEnglish
Pages (from-to)253-262
Number of pages10
JournalGeophysical Journal International
Volume126
Issue number1
DOIs
Publication statusPublished - Jul 1996

Keywords

  • Carbonates
  • Lofer cycles
  • Palaeomagnetism
  • Triassic

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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