Isuum

In the course of the redefinition of the Precambrian, the Isuum becomes the second period within the era of the Paleoarchean . It took 320 million years, from 3810 to 3490 million years BP .

designation

The name Isuum, engl. Isuan, or Isuan period , is derived from the Isua greenstone belt . This greenstone belt is located on the southwest coast of Greenland and belongs geologically to the North Atlantic Craton , more precisely to the Isukasia Terran .

Redefinition of the Precambrian Periods

Due to the moving away from period boundaries determined purely by radiometry, Gradstein et al. (2012) should now apply the GSSP principle as far as possible in the Precambrian as well . The periods are thus defined on the basis of significant geological events and no longer on arbitrary, radiometric ages. However, in the absence of supracrustal rock series, the lower limit of the isuum must necessarily be determined chronometrically.

Definition of Isuum

Geological map of the southwest coastal region of Greenland; Isua green stone belt (ISB) in green, Itsaq gneiss in brown and yellow border

The lower limit of the Isuum to the preceding first period of the Paleoarchean, the Acastum , is chronometrically set at 3810 million years BP. The upper limit of the Isuum to the subsequent Vaalbarum (and thus the Paleoarchean-Mesoarchean border ) marks a first GSSP that comes to lie at the lower edge of the Dresser Formation . The Dresser Formation, which belongs to the West Australian Warrawoona Group , concordantly overlays the cushion basalts of the North Star basalt . The GSSP can be assigned an age of 3490 million years.

meaning

With the Isuum , supracrustal rocks that have survived appear for the first time in the history of the earth . These exist as metasediments and contain the first, geochemically recognizable signatures of life .

Occurrence

• Greenland:
  • Itsaq Gneiss Complex - 3900 to 3550 million years BP
  • Isua greenstone belt of the Isukasia Terran - 3810 to 3700 million years BP
  • Akilia Association on Akilia Island - around 3850 million years BP (controversial)
• Labrador :
  • Uivak gneiss ; contain as inclusions the supracrustal volcanic rocks of Nulliak (Engl. Nulliak supracrustals ) - 3780 million years BP
• North Quebec :
  • Nuvvuagittuq greenstone belt of the superior craton ( Inukjuak terran of the Ungava peninsula ) - 3780 to 3750 million years BP
• Australia :
  • Narryer Gneiss Terran in the Yilgarn Craton - 3730 to 3450 million years BP
  • East Pilbara Terrane , Pilbara Kraton
    • Pilbara Supergroup with Warrawoona Group - 3525 to 3426 million years BP
      • Coonterunah Subgroup - 3525 to 3490 million years BP
        • Double bar formation
        • Coucal Formation - 3515 to 3498 million years BP
        • Table top formation
• South Africa :
  • Swaziland Supergroup of the Kaapvaal Craton
    • Onverwacht Group - 3547 to 3260 million years BP
      • Tjakastad subgroup - 3548 to 3472 million years BP
        • Theespruit Formation - 3548 to 3538 million years BP
        • Sandspruit Formation - 3540 to 3521 million years BP

Individual evidence

1. ^ Felix M. Gradstein et al .: On the Geologic Time Scale . In: Newsletters on Stratigraphy . tape 45/2 , 2012, p. 171-188 .
2. ^ Van Kranendonk, MJ et al .: Geological setting of Earth's oldest fossils in the c. 3.5 Ga Dresser Formation, Pilbara Craton, Western Australia . In: Precambrian Research . tape 167 , 2008, p. 93-124 .
3. ↑ Cates, NL, Ziegler, K., Schmitt, AK and Mojzsis, SJ: Reduced, reused and recycled: Detrital zircons define a maximum age for the Eoarchean (approx. 3750-3780Ma) Nuvvuagittuq Supracrustal Belt, Québec (Canada) . In: Earth Planet. Sci. Lett. tape 362 , 2013, p. 283-293 .