Vaalbarum

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As part of the redefinition of the Precambrian is Vaalbarum to the first period in the era of Mesoarchaikums . It took 470 million years, from 3490 to 3020 million years BP .

designation

The name Vaalbarum, engl. Vaalbaran, was derived from the supercontinent Vaalbara . Vaalbara is in turn a Portmanteau word made up of -vaal, the second part of the word Transvaal ( Transvaal-Kraton in South Africa ) and -bara, the last two syllables of Pilbara ( Pilbara-Kraton in Western Australia ).

Redefinition of the Precambrian Periods

In the course of moving away from period boundaries determined purely by radiometry, the GSSP principle should now be applied as far as possible in the Precambrian, according to Gradstein et al. (2012) . The periods are thus defined on the basis of significant geological events and no longer on arbitrary, radiometric ages.

Definition of the Vaalbarum

The lower limit of the Vaalbarum to the preceding Isuum (and thus the Paleoarchean-Mesoachean boundary ) is determined by a GSSP, which 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. The upper limit of the Vaalbarum to the second period of the Mesoarchean, the Pongolum , also marks a GSSP. This is located in the Gorge Creek Group ( De Gray Supergroup ), at the concordant contact between the basal conglomerate and the quartz-rich sandstone above it . Its age is 3020 million years BP.

meaning

Stromatolites of the approximately 3,400 million year old Strelley Pool Formation of the Western Australian Pilbara Craton

The main characteristic of the Vaalbarum is undoubtedly the first appearance of macroscopically recognizable life . The oldest known stromatolites appear at the base of the Dresser Formation . After the meteorite bombardment has finally subsided, their spread is linked to the emergence of the first stable continents (or continent germs) and their floating lithospheric roots. Good examples in this regard are the Kapvaal and Pilbara kratons. In addition to stromatolites, according to van Kranendonk (2010b) there are numerous other references to primitive life forms in the Vaalbarum.

The thread-like microfossils of the 3465 million year old Apex-Chert from the Pilbara craton were considered to be the second oldest evidence of life to date , after the finds from Isua and Akilia in southwest Greenland (which are only indirect, purely geochemical evidence). Recently, however, the biological origin of these filaments has been questioned, since such structures can also arise inorganically.

Recent research by Banerjee et al. (2006) point to tube-like structures that were created by microbes in the glass-like bark of pillow lavas and in hyaloclastites of the Onverwacht Group. Should the authenticity of these structures prove to be true, the place of origin of life would have to be found in the marine environment at the boundary layer to the oceanic crust .

Meteorite impacts

In the period from 3470 to 3240 million years BP, four horizons (S1 to S4) are deposited within the Swaziland supergroup of the South African Kaapvaal kraton , which indicate possible meteorite impacts . The lowest horizon can also be detected in the Warrawoona Group of the Western Australian Pilbara Kraton . The spherules in these layers are characterized by extremely high chromium and iridium values , which suggest an extraterrestrial origin. Typical signs of impact metamorphosis , however, are missing or can no longer be detected, so that its clear allocation to impact events remains controversial.

stratigraphy

Significant geological formations

Deposits

Magmatism

Geodynamics

Orogenesis

Individual evidence

  1. ^ Felix M. Gradstein et al .: On the Geologic Time Scale . In: Newsletters on Stratigraphy . tape 45 , no. 2 , 2012, p. 171-188 .
  2. ^ MJ Van Kranendonk, among others: 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. ^ MJ Van Kranendonk: Three and a half billion years of life on Earth: a transect back into deep time . In: Geological Survey of Western Australia . Record 2010/21, 2010, p. 93 .
  4. ↑ JW Schopf, among others: Laser-Raman imagery of Earth's oldest fossils . In: Nature . tape 416 , 2002, pp. 73-76 .
  5. ^ Garcia-JM Ruiz, among others: Self-assembled silica-carbonate structures and detection of ancient microfossils . In: Science . tape 302 , 2003, p. 1194-1197 .
  6. ^ NR Banerjee, inter alia: Preservation of 3.4-3.5 Ga microbial biomarkers in pillow lavas and hyaloclastites from the Barberton Greenstone Belt, South Africa . In: Earth and Planetary Science Letters . tape 241 , 2006, p. 707-722 .
  7. ↑ DR Lowe, inter alia: Spherule beds 3.47-3.24 billion years old in the Barberton Greenstone Belt, South Africa: A record of large meteorite impacts and their influence on early crustal and biological evolution . In: Astrobiology . v. 3, 2003, p. 7-47 .
  8. ^ DR Lowe, GR Byerly: Stratigraphy of the west-central part of the Barberton Greenstone Belt, South Africa . In: DR Lowe, GR Byerly (Ed.): Geologic evolution of the Barberton Greenstone Belt, South Africa. Geological Society of America Special Paper . vol. 329, 1999, pp. 1-36 .
  9. ^ Axel Hofmann: The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-Archaean times . In: Precambrian Research . tape 143 , no. 1-4 , December 15, 2005, pp. 23-49 , doi : 10.1016 / j.precamres.2005.09.005 .
  10. ^ SL Kamo, SW Davis: Reassessment of Archean crustal development in the Barberton Mountain Land, South Africa, based on U-Pb dating . In: Tectonics . tape 13 , 1994, pp. 167-192 .
  11. ^ JW Goodge, CM Fanning: 2.5 billion years of punctuated Earth history as recorded in a single rock . In: Geology . tape 27 , 1999, p. 1007-1010 .
  12. ↑ JW Goodge, among others: U-PB evidence of 1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions . In: Precambrian Research . tape 112 , 2001, p. 261-288 .
  13. ^ MJ Robertson, among others: Gold mineralization during progressive deformation at the Main Reef Complex, Sheba Gold Mine, Barberton Greenstone Belt, South Africa . In: Africa. Econ. Geol. Res. Unit Inf. Circ. tape 267 . University of Witwatersrand, 1993, p. 1-26 .
  14. ^ SA De Waal: Nickel minerals from Barberton, South Africa. VII. The spinels Co-chromite and Ni-chromite and their significance for the origin of the Bon Accord nickel deposit . In: Bull. BRGM Band II , no. 2 , 1978, p. 223-230 .