Dresser formation

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The dresser formation is the final formation of the Talga Talga subgroup in Western Australia ( Pilbara-Kraton ) belonging to the Warrawoona Group . With an age of 3490 to 3480 million years BP , it comes from the Paleoarchean . Its lower edge marks the first GSSP to mark the border between Isuum and Vaalbarum ( Paleoarchean-Mesoarchean border ). The formation contains the oldest known macrofossils in the form of stromatolites and other indications of microbial life.

Recent stromatolites in Yalgorup National Park in Western Australia

Occurrence

The Dresser Formation occurs only in an area of ​​25 square kilometers in the panoramic greenstone belt of the East Pilbara Terrane . Here it forms part of the dome-like bulge of the North Pole Dome . As a ring of concentric mounds, it surrounds the central intrusion of the 3460 to 3420 million year old North Pole Monzogranite with a diameter of 14 kilometers . Due to the intrusion, the layers fall outwards relatively flat.

stratigraphy

The dresser formation follows the pillow lavas of the North Star basalt . It is in turn overlaid discordantly by Mount Ada Basalt or the Duffer Formation (both Coongan subgroups ). It correlates sideways with the McPhee formation .

The approximately 1000 meter thick Dresser Formation begins with a 4 to 60 meter thick sedimentary chert layer , which is underlain by Spinifex-bearing metabasalts . The metabasalts have been intensively hydrothermally changed and are interspersed with innumerable veins of black and gray silica , silica with barite and barite in the centimeter to kilometer range . This is joined by pyrite in different proportions.

The fossil-bearing chert layer is mainly made up of gray, white, black and locally red cherts. It contains mighty, often meter-thick horizons made of coarse crystalline barite, which occur both concordantly and discordantly to the stratification. Conglomerates and sandstones as well as carbonates and stromatolite laminates follow .

On top of that I then lay acidic and mafic / ultramafic volcanoclastics as well as the main mass of the formation consisting of komatiitic cushion basalts and dolerite deposits . Up to four more lens-shaped layers of chert ± barite ± carbonate ± jasper are integrated into the pillow basalts .

Fossil content

In the fossil record , the Dresser Formation is the oldest formation in which macrofossils can be recognized. These are stromatolites that appear for the first time within the Chert-Baryt unit. The stromatolites have a pleated lamination and show stratiform (mat), column, dome and cone structures. The cone structures occur in chert and are characterized by neat layering, which can even be organized as ripples . The interpretation of these structures as stromatolites of biogenic origin is still not shared by all authors - for example, they are referred to as stromatoloids or possible stromatolites or represented as purely inorganic formations resulting from deformation processes (folds).

Furthermore, clasts with laminated, carbon-rich material can be found in layered micrites , which are interpreted as eroded microbe mats . Noffke and colleagues (2013) also come to a similar result, who in their detailed investigation of the sedimentary structures of the Dresser Formation interpret a large part of the structures found as being caused by microbes ( microbially induced sedimentary structures or MISS for short ). The found bandwidth of the structures, they arrange a Sabcha to -Ablagerungsmilieu which they divide into a further subtidales, tidales and supratidales Environment. In addition, you can see a lagoon and bar area (English barrier shoal ).

Dunlop and colleagues (1978) were also able to describe carbon-containing spheroids and filaments in the Dresser formation, which they also interpreted as microfossils - whereas Buick (1990) advises caution with this view. Ueno and colleagues were also able to find carbon-containing filaments in the silica-containing veins below the chert-barite unit, but their biogenicity is also controversial. Nevertheless, methane fluid inclusions indicate microbial methane production.

Deposit conditions

Van Kranendonk and colleagues (2008) interpret the Dresser Formation as deposits in a subsidiary submarine caldera above an intrusive magma chamber. Hydrothermal processes played a very important role here. Quiet lagoons or mid-ocean ridges have been suggested as alternative deposition environments . The numerous tectonic disturbances of the Dresser formation speak against the lagoon model. The suffered hydrothermal changes based on acid-sulphate reactions, as well as the proven shallow water to surface conditions and the presence of volcanic material contradict the milieu of the oceanic ridges.

In the Dresser Formation, calm deposition conditions, in which iron- rich carbonates were precipitated, alternate with restless periods of intense hydrothermal activity and the extraction of acidic volcanic rocks. The turbulent times are also of tectonic instability and the development of growth-dislocations ( engl. Growth faults ) embossed.

The carbonate precipitation reached its peak under shallow water conditions (up to occasional drying out) - ideal conditions for the growth of the stromatolites. Then the main phase of caldera formation began, which was accompanied by a very strong circulation of hydrothermal fluids. Coarse conglomerates and fault blocks formed, the sediments were partially deformed while still unconsolidated, and an erosion discordance also arose. The final collapse of the caldera eventually resulted in the sedimentation of sandstones and micritic carbonates.

Hydrothermal changes

The sunken lying blocks of the Dresser Formation were hydrothermally changed very strongly. Areas with propylitic (greening), argillic and advanced argillic transformations (new formation of clay minerals) indicate acid-sulphate reactions in the shallow marine area under the action of hot water vapor. The transition temperatures reached 250 to 300 ° C at the base, but only 50 to 80 ° C at the surface where the hydrothermal aragonite was precipitated. The crystallization of the barite took place through several hydrothermal pulses both in the veins of the changed lying area and as carbonate displacement in the sediment sequence. Evaporative sulfate precipitation is rarely observed - however, plaster rosettes are suspected below the stromatolites.

metamorphosis

Due to the penetration of the North Pole Monzogranite , the rocks in the central area of ​​the North Pole Dome succumbed to a metamorphosis that took place under the physical conditions of the amphibolite facies . Further outside, only the conditions of the prehnite - pumpellyite - to green schist facies were realized.

Dating

Thorpe and colleagues found an age of 3490 million years BP in the galena of the Chert-Baryt unit using the lead-lead method. Van Kranendonk and colleagues (2008) were able to determine an age of 3481 million years BP for a volcanoclastic sandstone, the area of ​​origin of the clasts having an age of formation of 3525.3 ± 1.8 million years BP.

meaning

The Dresser Formation, together with the somewhat older Coucal Formation, contains the oldest, best preserved and only weakly metamorphosed volcanic and sedimentary rocks on earth. The stromatolite and microbial mat structures encountered make it the oldest formation in the fossil record, in which primitive life appears with very high probability for the first time. For this reason, the lower edge of the Dresser Formation is also proposed as the first GSSP to mark the Paleoarchean-Mesoarchean boundary. The next younger stromatolites can be found in the Strelley Pool Chert of the Warrawoona Group and the next younger microbial mats in the around 3,200 million year old Moodies Group in South Africa . Secured filaments and spheroids are added from the Strelley Pool Chert - the finds in the Apex Chert of the Apex Basalt are still controversial (Brasier-Schopf debate).

Individual evidence

  1. ↑ Tara Djokic et al .: Earliest signs of life on land preserved in approx. 3.5 Ga hot spring deposits. In: Nature Communications. Volume 8, Article No. 15263, 2017, doi: 10.1038 / ncomms15263
    Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks. On: eurekalert.org from May 9, 2017
  2. ↑ a b c d M. J. Van Kranendonk, among others: Geological setting of Earth's oldest fossils in the ca.3.5 Ga Dresser Formation, Pilbara Craton, Western Australia . In: Precambrian Research . tape 167 , 2008, p. 93-124 , doi : 10.1016 / j.precamres.2008.07.003 .
  3. ^ MJ Van Kranendonk, F. Pirajno: Geological setting and geochemistry of metabasalts and alteration zones associated with hydrothermal chert ± barite deposits in the 3.45 Ga Warrawoona Group, Pilbara Craton, Australia . In: Geochemistry: Exploration, Environment, Analysis . tape 4 , 2004, p. 253-278 .
  4. ^ MJ Van Kranendonk: Geology of the North Shaw 1/100000 sheet . Western Australia Geological Survey, 1: 100000 series explanatory notes, 2000.
  5. ^ W. Nijman, inter alia: Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia (Part 2) . In: Precambrian Research . tape 95 , 1999, pp. 247-274 .
  6. ^ MR Walter, inter alia: Stromatolites, 3400-3500 Myr old from the North Pole Area, Western Australia . In: Nature . tape 284 , 1980, pp. 443-445 .
  7. ^ R. Buick, et al: Stromatolite recognition in ancient rocks: an appraisal of irregular laminated structures in an early Archaean chert-barite unit from North Pole, Western Australia . In: Alcheringa . tape 5 , 1981, pp. 161-181 .
  8. ↑ DR Lowe: Abiological Described origin of stromatolites older than 3.2 Ga . In: Geology . tape 22 , 1994, pp. 387-390 .
  9. ^ N. Noffke, inter alia: Microbially induced sedimentary structures recording an ancient ecosystem in the approx.3.48 billion year-old Dresser Formation, Pilbara, Western Australia . In: Astrobiology . tape 13 , no. 12 , 2013, p. 1103–1124 , doi : 10.1089 / ast.2013.1030 .
  10. ^ JSR Dunlop, among others: A new microfossil assemblage from Western Australia . In: Nature . tape 274 , 1978, pp. 676-678 .
  11. ↑ R. Buick: Micro Fossil recognition in Archean rocks: an appraisal of spheroids and filaments from a 3500 MY old chert-barite unit at North Pole, Western Australia . In: Palaios . tape 5 , 1990, pp. 441-459 .
  12. ↑ Y. Ueno, inter alia: Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era . In: Nature . tape 440 , 2006, pp. 516-519 .
  13. ^ DI Groves, among others: An early habitat of life . In: Scientific American . tape 245 , 1981, pp. 64-73 .
  14. ↑ K. Kitajima, inter alia: Seafloor hydrothermal alteration at an Archaean mid-ocean ridge . In: Journal of Metamorphic Geology . tape 19 , 2001, p. 581-597 .
  15. ^ RI Thorpe, inter alia: Constraints to models for Archaean lead evolution from precise U-Pb geochronology from the Marble Bar region, Pilbara Craton, Western Australia . In: JE Glover, Susan E. Ho (Eds.): The Archaean: Terrains, Processes and Metallogeny . Publication 22. Geology Department and University Extension, The University of Western Australia, 1992, pp. 395-408 .
  16. ^ A. Hickman: Review of the Pilbara Craton and the Fortescue Basin, Western Australia: crustal evolution providing environments for early life . In: Island Arc . tape 21 , 2012, p. 1-31 .
  17. ^ C. Heubeck: An early ecosystem of Archean tidal microbial mats (Moodies Group, South Africa, approx. 3.2 Ga) . In: Geology . tape 37 , 2009, p. 931-934 .