Pilbara Craton

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Pilbara Craton
Stratigraphic range: Eoarchean to Paleoproterozoic [1]
Pilbara craton from satellite.jpg
Satellite view in 2013 of Pilbara Craton
Type Geological formation
AreaEstimated 250,000 km2 (97,000 sq mi), [1] Pilbara IRBA v7 region 178,231.26 km2 (68,815.47 sq mi) [2]
Thicknessup to 20 km (12 mi)
Lithology
Primary Granite
Other Greenstone
Location
Region Western Australia
Country Australia
Type section
Named for Pilbara
Named bySee Pilbara#Etymology
IBRA 6.1 Pilbara.png
Map of Australia with the Pilbara region highlighted in red.
Interim Biogeographic Regionalisation for Australia, version 7.pdf
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KEY
Pilbara Craton subregions:
PIL01 -
Chichester
PIL02 -
Fortescue
PIL03 -
Hamersley
PIL04 -
Roebourne
Carnarvon subregion:
CAR01 -
Cape Range
Gascoyne subregion:
GAS01 -
Ashburton
Interim Biogeographic Regionalisation for Australia, version 7.pdf
Pilbara Craton part of the continental lithosphere using version 7 of IBRA classification. The geo-ecosystems of the PIL02(Fortescue) area in particular are usually on much younger exposed rock formations (< 1.7 Ga) than the lithology discussed in this article.

The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia.

Contents

The Pilbara Craton is one of only two pristine Archaean 3.8–2.7 Ga (billion years ago) crusts identified on the Earth, along with the Kaapvaal Craton in South Africa. The youngest rocks are 1.7 Ga old in the historic area assigned to the Craton. [1] Both locations may have once been part of the Vaalbara supercontinent or the continent of Ur.

There are two subregional geographical classification regimes used, being:

  1. The Interim Biogeographic Regionalisation for Australia based upon interacting geo-ecosystems
  2. Based on geology alone where the eastern continuous oldest portion is called the Eastern Pilbara Craton and younger surface lithologies within the larger craton have different names.
The currently exposed continuous Pilbara Craton in red, the Eastern Pilbara region outlined in blue, and detail of local lithologies. However this map does not show other discontinuous exposed oldest rocks of the Pilbara Craton. Accordingly a reader should refer to the references for more detailed geological mapping which is not reproduced here for copyright reasons. Pilbara Craton Region Map.pdf
The currently exposed continuous Pilbara Craton in red, the Eastern Pilbara region outlined in blue, and detail of local lithologies. However this map does not show other discontinuous exposed oldest rocks of the Pilbara Craton. Accordingly a reader should refer to the references for more detailed geological mapping which is not reproduced here for copyright reasons.

Geology

The most important part of the Pilbara Craton to understand the early Earth crust is called the Eastern Pilbara Craton, where still exposed today, are crustal rocks that are up to 3.8 billion years old and intrusive granitic domes along with greenstone belts that are about 3.5 to 3.2 billion years old. [1] The geology was reassessed in 2007 with the separation out from the geologically named Pilbara Craton of a thick succession of interbedded clastic or chemical sedimentary rocks and volcanic rocks forming the Fortescue, Hamersley, and Turee Creek basins that are usually aged from 2.78–2.42 billion years old and the younger volcano-sedimentary Ashburton Basin aged from 2.21–1.79 billion years ago. [1] A surface region between the Fortescue and Hamersley basins is even younger, at less than 1.7 billion years old, as are the surrounding geo-ecosystems surface rocks to the Pilbara Craton. It is important to note that to the east and south of the Eastern Pilbara Craton there are significant outcrops of the very old rocks and that these are confined to the traditional area of the Pilbara Craton which is inferred to be subsurface for more than half its area. [1]

Mineralogy

There are extensive high quality iron ore deposits and also economic to mine gold, silver, copper, nickel, lead, zinc, molybdenum, vanadium and fluorite deposits. [1]

Evidence of earliest life

Evidence of the earliest known life on land may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Dresser Formation in the Pilbara Craton. [3] [4] [5] Biogenic sedimentary structures (microbialites) such as stromatolites and MISS were described from tidal, lagoonal and subtidal coastal settings that can be reconstructed from the Dresser stratigraphy as well. [6] The rocks of the Dresser Formation display evidence of haematite alteration that may have been microbially influenced. [7]

Apex chert Apex Chert.jpg
Apex chert

The earliest direct evidence of life on Earth may be fossils of microorganisms permineralized in 3.465-billion-year-old Australian Apex chert rocks. [8] [9] However, the evidence for the biogenicity of these microstructures has been thoroughly debated. [10] [11] Originally, 11 taxa were described from a deposit thought to be located at the mouth of a river due to certain characteristics like rounded and sorted grains. [12] [13] Extensive field mapping and petrogenetic analysis has since shown the setting for the purported microfossils to be hydrothermal [14] [15] and this is widely supported. [16] [17] [18] [19] Consequently, many alternative abiotic explanations have been proposed for the filamentous microstructures including carbonaceous rims around quartz spherules and rhombs, [14] [15] witherite self-assembled biomorphs [20] and haematite infilled veinlets. [21] The carbonaceous matter composing the filaments has also been repeatedly examined with Raman spectroscopy [14] [22] [21] which has yielded mixed interpretations of results and is therefore regarded by many to be unreliable for determining biogenicity when used alone. [23] [24] Perhaps the most compelling argument to date is based on high spatial resolution electron microscopy like scanning and transmission electron microscopy. [19] This study concludes that the nano-scale morphology of the filaments and the distribution of the carbonaceous matter are inconsistent with a biological origin for the filaments. Instead, it is more likely that the hydrothermal conditions have assisted in the heating, hydration and exfoliation of potassium micas on which barium, iron and carbonate have secondarily been adsorbed.

Carbonaceous structures appearing to be of biological origin have also been discovered in the 3.47 billion year-old Mount Ada Basalt, a rock layer that is a few million years older than the Apex chert. However, the biogenicity of these supposed fossils has also been disputed, with some studies finding abiotic processes to be a more likely culprit for their formation. [11]

Additional potential bioindicators from the Precambrian have been found in the region, including carbonaceous microfossils in the northeastern Pilbara Craton. [25]

See also

Related Research Articles

<span class="mw-page-title-main">Archean</span> Geologic eon, 4031–2500 million years ago

The Archean Eon, in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history, preceded by the Hadean Eon and followed by the Proterozoic. The Archean represents the time period from 4,031 to 2,500 Ma. The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The Huronian glaciation occurred at the end of the eon.

<span class="mw-page-title-main">Chert</span> Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

<span class="mw-page-title-main">Marble Bar, Western Australia</span> Town in Western Australia

Marble Bar is a town and rock formation in the Pilbara region of north-western Western Australia. It was the social centre of European settlers in the Pilbara region during the early 1900s, predating the construction of other towns now established.

<span class="mw-page-title-main">Paleoarchean</span> Second era of the Archean Eon

The Paleoarchean, also spelled Palaeoarchaean, is a geologic era within the Archean Eon. The name derives from Greek "Palaios" ancient. It spans the period of time 3,600 to 3,200 million years ago. The era is defined chronometrically and is not referenced to a specific level of a rock section on Earth. The earliest confirmed evidence of life comes from this era, and Vaalbara, one of Earth's earliest supercontinents, may have formed during this era.

<span class="mw-page-title-main">Gunflint chert</span> Geologic formation in Minnesota and Ontario

The Gunflint chert is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic. The Gunflint Chert is composed of biogenic stromatolites. At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite colonies of cyanobacteria that have converted to jasper are found in Ontario. The banded ironstone formation consists of alternating strata of iron oxide-rich layers interbedded with silica-rich zones. The iron oxides are typically hematite or magnetite with ilmenite, while the silicates are predominantly cryptocrystalline quartz as chert or jasper, along with some minor silicate minerals.

Biotic material or biological derived material is any material that originates from living organisms. Most such materials contain carbon and are capable of decay.

<span class="mw-page-title-main">Paleobiology</span> Study of organic evolution using fossils

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<span class="mw-page-title-main">Vaalbara</span> Archaean supercontinent from about 3.6 to 2.7 billion years ago

Vaalbara is a hypothetical Archean supercontinent consisting of the Kaapvaal Craton and the Pilbara Craton. E. S. Cheney derived the name from the last four letters of each craton's name. The two cratons consist of continental crust dating from 2.7 to 3.6 Ga, which would make Vaalbara one of Earth's earliest supercontinents.

Elso Sterrenberg Barghoorn was an American paleobotanist, called by his student Andrew Knoll, the present Fisher Professor of Natural History at Harvard, "the father of Pre-Cambrian palaeontology."

<span class="mw-page-title-main">Warrawoona Group</span>

The Warrawoona Group is a geological unit in Western Australia containing putative fossils of cyanobacteria cells. Dated 3.465 Ga, these microstructures, found in Archean chert, are considered to be the oldest known geological record of life on Earth.

This timeline of natural history summarizes significant geological and biological events from the formation of the Earth to the arrival of modern humans. Times are listed in millions of years, or megaanni (Ma).

<span class="mw-page-title-main">Martin Brasier</span> British paleontologist (1947–2014)

Martin David Brasier FGS, FLS was an English palaeobiologist and astrobiologist known for his conceptual analysis of microfossils and evolution in the Precambrian and Cambrian.

<span class="mw-page-title-main">Archean life in the Barberton Greenstone Belt</span> Some of the most widely accepted fossil evidence for Archean life

The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in the Barberton fossil record in rocks as old as 3.5 billion years. The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks.

James William Schopf is an American paleobiologist and professor of earth sciences at the University of California Los Angeles. He is also Director of the Center for the Study of Evolution and the Origin of Life, and a member of the Department of Earth and Space Sciences, the Institute of Geophysics and Planetary Physics, and the Molecular Biology Institute at UCLA. He is most well known for his study of Precambrian prokaryotic life in Australia's Apex chert. Schopf has published extensively in the peer reviewed literature about the origins of life on Earth. He is the first to discover Precambrian microfossils in stromatolitic sediments of Australia (1965), South Africa (1966), Russia (1977), India (1978), and China (1984). He served as NASA's principal investigator of lunar samples during 1969–1974.

<span class="mw-page-title-main">Eastern Pilbara Craton</span> Carton in Western Australia

The Eastern Pilbara Craton is the eastern portion of the Pilbara Craton located in Western Australia. This region contains variably metamorphosed mafic and ultramafic greenstone belt rocks, intrusive granitic dome structures, and volcanic sedimentary rocks. These greenstone belts worldwide are thought to be the remnants of ancient volcanic belts, and are subject to much debate in today's scientific community. Areas such as Isua and Barberton which have similar lithologies and ages as Pilbara have been argued to be subduction accretion arcs, while others suggest that they are the result of vertical tectonics. This debate is crucial to investigating when/how plate tectonics began on Earth. The Pilbara Craton along with the Kaapvaal Craton are the only remaining areas of the Earth with pristine 3.6–2.5 Ga crust. The extremely old and rare nature of this crustal region makes it a valuable resource in the understanding of the evolution of the Archean Earth.

<span class="mw-page-title-main">Earliest known life forms</span> Putative fossilized microorganisms found near hydrothermal vents

The earliest known life forms on Earth may be as old as 4.1 billion years old according to biologically fractionated graphite inside a single zircon grain in the Jack Hills range of Australia. The earliest evidence of life found in a stratigraphic unit, not just a single mineral grain, is the 3.7 Ga metasedimentary rocks containing graphite from the Isua Supracrustal Belt in Greenland. The earliest direct known life on land may be stromatolites which have been found in 3.480-billion-year-old geyserite uncovered in the Dresser Formation of the Pilbara Craton of Western Australia. Various microfossils of microorganisms have been found in 3.4 Ga rocks, including 3.465-billion-year-old Apex chert rocks from the same Australian craton region, and in 3.42 Ga hydrothermal vent precipitates from Barberton, South Africa. Much later in the geologic record, likely starting in 1.73 Ga, preserved molecular compounds of biologic origin are indicative of aerobic life. Therefore, the earliest time for the origin of life on Earth is at least 3.5 billion years ago, possibly as early as 4.1 billion years ago — not long after the oceans formed 4.5 billion years ago and after the formation of the Earth 4.54 billion years ago.

<span class="mw-page-title-main">Evolution of bacteria</span> Development of bacteria throughout time

The evolution of bacteria has progressed over billions of years since the Precambrian time with their first major divergence from the archaeal/eukaryotic lineage roughly 3.2-3.5 billion years ago. This was discovered through gene sequencing of bacterial nucleoids to reconstruct their phylogeny. Furthermore, evidence of permineralized microfossils of early prokaryotes was also discovered in the Australian Apex Chert rocks, dating back roughly 3.5 billion years ago during the time period known as the Precambrian time. This suggests that an organism in of the phylum Thermotogota was the most recent common ancestor of modern bacteria.

The Dresser Formation is a Paleoarchean geologic formation that outcrops as a generally circular ring of hills the North Pole Dome area of the East Pilbara Terrane of the Pilbara Craton of Western Australia. This formation is one of many formations that comprise the Warrawoona Group, which is the lowermost of four groups that comprise the Pilbara Supergroup. The Dresser Formation is part of the Panorama greenstone belt that surrounds and outcrops around the intrusive North Pole Monzogranite. Dresser Formation consists of metamorphosed, blue, black, and white bedded chert; pillow basalt; carbonate rocks; minor felsic volcaniclastic sandstone and conglomerate; hydrothermal barite; evaporites; and stromatolites. The lowermost of three stratigraphic units that comprise the Dresser Formation contains some of the Earth's earliest commonly accepted evidence of life such as morphologically diverse stromatolites, microbially induced sedimentary structures, putative organic microfossils, and biologically fractionated carbon and sulfur isotopic data.

<span class="mw-page-title-main">Silicification</span> Geological petrification process

In geology, silicification is a petrification process in which silica-rich fluids seep into the voids of Earth materials, e.g., rocks, wood, bones, shells, and replace the original materials with silica (SiO2). Silica is a naturally existing and abundant compound found in organic and inorganic materials, including Earth's crust and mantle. There are a variety of silicification mechanisms. In silicification of wood, silica permeates into and occupies cracks and voids in wood such as vessels and cell walls. The original organic matter is retained throughout the process and will gradually decay through time. In the silicification of carbonates, silica replaces carbonates by the same volume. Replacement is accomplished through the dissolution of original rock minerals and the precipitation of silica. This leads to a removal of original materials out of the system. Depending on the structures and composition of the original rock, silica might replace only specific mineral components of the rock. Silicic acid (H4SiO4) in the silica-enriched fluids forms lenticular, nodular, fibrous, or aggregated quartz, opal, or chalcedony that grows within the rock. Silicification happens when rocks or organic materials are in contact with silica-rich surface water, buried under sediments and susceptible to groundwater flow, or buried under volcanic ashes. Silicification is often associated with hydrothermal processes. Temperature for silicification ranges in various conditions: in burial or surface water conditions, temperature for silicification can be around 25°−50°; whereas temperatures for siliceous fluid inclusions can be up to 150°−190°. Silicification could occur during a syn-depositional or a post-depositional stage, commonly along layers marking changes in sedimentation such as unconformities or bedding planes.

References

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Bibliography