Diagenesis in platform carbonate rocks: a palaeomagnetic study of an upper Triassic-lower Jurassic section, Tata (Hungary)

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Abstract

This paper is part of the special publication No.151, Palaeomagnetism and diagenesis in sediments (eds D.H. Harling and P. Turner). The upper Triassic platform carbonate rocks at Tata are penetrated by three generations of neptunean dykes and cut by strike-slip faults connected to the platform's break-up in early Jurassic time. These platform carbonate rocks are conformably overlain by lower Jurassic pelagic carbonate rocks, with a clear change in sedimentation type at the Triassic-Jurassic boundary. Openings within the Triassic platform, including bedding-conformable cavities and Megalodon shells, are filled with Jurassic material showing late 'diagenetic' features with respect to the Triassic platform facies. The Triassic platform carbonate rocks were examined in a 10m high outcrop and in a 200m core, which commenced 40m below the surface. The Triassic rock commonly has a weak initial natural remanent magnetization (NRM) and diamagnetic susceptibilities, in both outcrop and within the core, and has the same kind of magnetic mineralogy as in the 'diagenetic' material and stratified Jurassic rock, but the magnetic mineral content is higher in the Jurassic materials. The core has mixed polarity, whereas the Triassic outcrop and Jurassic materials have only normal polarity. The overall mean palaeomagnetic direction for the core is similar to those in two large quarries of similar age in the same area (the Gerecse Hills), but differs significantly from the overall mean directions of both the Jurassic 'diagenetic' material and the Jurassic beds, which are practically identical to each other. Some sites and individual samples in the Triassic outcrop carbonate rocks show Jurassic directions, but others show remagnetization circle distributions between this Jurassic direction and the present-day Earth's magnetic field. These observations suggest that the remanence of the latest Triassic time was reset during the early Jurassic diagenesis, but this was not associated with chemical changes as the mineralogies in all groups are the same.

Original languageEnglish
Pages (from-to)157-165
Number of pages9
JournalGeological Society Special Publication
Volume151
DOIs
Publication statusPublished - nov. 6 1999

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ASJC Scopus subject areas

  • Water Science and Technology
  • Ocean Engineering
  • Geology

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