# Geochronology/Precambrian

Def. "the time and geology dated before the Phanerozoic" or the "eon (or supereon) and rock formations dated before 541.0±1.0 million years ago, coinciding with the first appearance of the fossils of hard-shelled animals" is called the Precambrian.
Usage notes
The International Commission on Stratigraphy, which attempts to standardize the vocabulary of the field, is revising the boundaries between time periods based on physical-science methods rather than the kinds of fossils present.
The boundary between the Precambrian and the Phanerozoic has been changed from time to time and will be subject to change.

## Periods

Precambrian :
| Name (English) | base/start (Ma) | top/end (Ma) | status | subdivision of | usage | named after | author, year |
| Adelaidean | 1,300 | 542 | age | Proterozoic | Australia | Adelaide | |
| Aimchanian | | 1100 | age | Proterozoic | Siberia | | |
| Algonkian | | 543 | age | Proterozoic | international | Algonquian native peoples of Canada | |
| Amazonian | ~1,800 | present | Martian epoch | Martian epoch | Mars | Amazonis Planitia | |
| Animikean | 2,225 | 1,400 | age | Proterozoic | North America (obsolete) | | |
| Aphebian | 2500 | 1600 | age | Proterozoic | North America | | |
| Archean | none | 2,500 | eon | Precambrian | ICS | | |
| Azoic | | | eon | Precambrian | | | |
| Baikalian | 850 | 650 | age | Proterozoic | Siberia | Lake Baikal | |
| Basin Groups 1-9 | 4,150 | 3,850 | subperiod | Prenectarium | Moon (unofficial) | groups of impact basins | |
| Brioverian | ~680 | ~600 | age | Neoproterozoic | Armorican Massif, France | | |
| Burzyan | 1,400 | 1,375 | age | Proterozoic | Russia | | |
| Calymmian | 1,600 | 1,400 | period | Proterozoic | ICS | | |
| Carpentarian | 1,800 | 1,300 | age | Proterozoic | Australia | Gulf of Carpentaria | |
| Cryogenian | 850 | 635.5 ± 1.2 | period | Proterozoic | ICS | frozen beginning | |
| Cryptic | 4,567 | 4,150 | epoch | Prenectarian | Moon (unofficial) | hidden | |
| Early Imbrian | 3850 | 3800 | period | | Moon | Mare Imbrium | |
| Ectasian | 1,400 | 1,200 | period | Proterozoic | ICS | | |
| Ediacaran | 635.5 ± 1.2 | 542.0 ± 1.0 | period | Proterozoic | ICS | Ediacara Hills (Australia) | |
| Eoarchean | none | 3.600 | era | Archean | ICS | | |
| Eratosthenian | 3,200 | 1,100 | period | | Moon | Eratosthenes | |
| Fupingan | 3,100 | 2,600 | age | Archaean | China | | |
| Hadean | none | 4000 | eon | Precambrian | ICS | Hades, hell | Cloud, 1972 |
| Hadrynian | 850 | 542 | age | Neoproterozoic | North America | | |
| Helikian | 1,600 | 850 | age | Proterozoic | North America | | |
| Hesperian | ~3,500 | ~1,800 | Martian epoch | | Mars | Hesperia Planum | |
| Huronian | 2,500 | 1,400 | age | Proterozoic | worldwide (obsolete) | | |
| Imbrian | 3,850 | 3,200 | period | | Moon | Mare Imbrium | |
| Isuan | 3,800 | 3,500 | age | Archaean | Europe | | |
| Jinningian | 1,750 | 800 | age | Proterozoic | China | | |
| Karatau | 1,100 | 800 | age | Proterozoic | Russia | | |
| Luliangian | 2,350 | 1,750 | age | Proterozoic | China | | |
| Mayanan | 1100 | 850 | age | Proterozoic | Siberia | | |
| Mesoarchean | 3,200 | 2,800 | era | Archean | ICS | | |
| Mesoproterozoic | 1,600 | 1,000 | era | Proterozoic | ICS | | |
| Mokolian | 2,050 | 900 | age | Proterozoic | South Africa | | |
| Namibian | 900 | 542 | age | Neoproterozoic | South Africa | Namibia | |
| Nectarian | 3920 | 3850 | period | | Moon | Mare Nectaris | |
| Neoarchean | 2,800 | 2,500 | era | Archean | ICS | | |
| Neoproterozoic | 1,000 | 542.0 ± 1.0 | era | | ICS | | |
| Noachian | none | ~3,500 | Martian epoch | | Mars | Noachis Terra | |
| Nullaginian | 2,500 | 1,800 | age | Proterozoic | Australia | | |
| Orosirian | 2,050 | 1,800 | period | Proterozoic | ICS | | |
| Paleoarchean | 3,600 | 3,200 | era | Archean | ICS | | |
| Paleoproterozoic | 2,500 | 1,600 | era | Proterozoic | ICS | | |
| Precambrian | none | 542.0 ± 1.0 | none (before: eon) | | worldwide | before the Cambrian | |
| Prenectarian | 4567 | 3850 | period | | Moon | before the Nectarian | |
| Proterozoic | 2,500 | 542.0 ± 1.0 | eon | | ICS | | |
| Randian | 3,000 | 2,500 | age | Archaean | South Africa | | |
| Rhyacian | 2,300 | 2,050 | period | Proterozoic | ICS | | |
| Riphean | 1,650 | 650 | age | Proterozoic | worldwide (obsolete) | | |
| Siderian | 2,500 | 2,300 | period | Proterozoic | ICS | | |
| Sinian | 800 | 542 | age | Neoproterozoic | China | | |
| Statherian | 1,800 | 1,600 | period | Proterozoic | ICS | | |
| Stenian | 1,200 | 1,000 | period | Proterozoic | ICS | | |
| Sturtian | ~730 | | age | Neoproterozoic | worldwide, unofficial | | |
| Swazian | 4,000 | 3,000 | age | Archaean | South Africa | | |
| Tonian | 1,000 | 850 | period | Proterozoic | ICS | | |
| Vaalian | 2,500 | 2,050 | age | Proterozoic | South Africa | | |
| Vendian | ~610 | 542.0 ± 1.0 | subera | Proterozoic | worldwide (obsolete) | | |
| Wutaian | 2,600 | 2,350 | age | Archaean-Proterozoic | China | | |
| Yurmatian | 1,375 | 1,100 | age | Proterozoic | Russia | | |

## Proterozoic

Def. "the eon from 2,500 Ma to 541.0±1.0 Ma, the beginning of the Phanerozoic, marked by the build up of oxygen in the atmosphere and the emergence of primitive multicellular life" is called the Proterozoic.

## Upper Adelaidean

The Adelaidean appears to encompass the Delamerian Granites and the Adelaide Rift Complex.
"The deposits include the type sections for the often globally correlated Sturtian and Marinoan glacial sequences (e.g., Preiss, 2000) and the Global Stratotype Section and Point (GSSP) for the newly defined Ediacaran Period (Knoll et al., 2004)."
The later Adelaidean includes the Burra and Caliana Groups.

## Neoproterozoic

Def. "a geologic era within the Proterozoic eon; comprises the Tonian, Cryogenian and Ediacaran periods from about 1000 to 544 million years ago, when algae and sponges flourished" is called the Neoproterozoic.

## Ediacaran

"In the central Flinders Ranges the 4.5 km thick Umberatana Group encompasses the two main phases of glacial deposition (see Thomas et al., 2012). The carbonaceous, calcareous and pyritic Tindelpina Shale Member, of the interglacial Tapley Hill Formation, caps the Fe-rich diamictite and tillite formations of the Sturt glaciation. The upper Cryogenian glacials of the Elatina Formation are truncated by the Nuccaleena Formation at the base of the Wilpena Group and the Ediacaran System."
"In 2004, the Global Stratotype Section and Point (GSSP) for the terminal Proterozoic was placed near the base of the Nuccaleena Formation in Enorama Creek in the central Flinders Ranges [in the image on the right], thus establishing the Ediacaran System and Period (Knoll et al., 2006). As the Nuccaleena Formation has not been accurately dated, a date of c. 635 Ma from near-correlative levels in Namibia and China is presumed for the base of the Ediacaran (Hoffmann et al., 2004; Condon et al., 2005; Zhang et al., 2005)."
Def. "a geologic period within the Neoproterozoic era from about 620 to 542 million years ago" is called the Ediacaran.

## Gucheng

The Gucheng is apparently comparable to the Marinoan.

## Jiangkou

The Jiangkou spans the Chang'an through the Gucheng.

## Chang'an

The Chang'an occurred about 715.9 ± 2.8 Ma.

## Numees

The Numees has a Sturtian age.

## Tereeken

The Tereeken occurred < 727 ± 8 Ma.

## Port Nolloth

The Port Nolloth extends from the Kaigas formation upwards to the Murmees.

## Kaigas formation

The Kaigas glaciation was a hypothesized snowball earth event in the Neoproterozoic Era, preceding the Sturtian glaciation inferred based on the interpretation of Kaigas Formation conglomerates in the stratigraphy overlying the Kalahari Craton as correlative with pre-Sturtian Numees formation glacial diamictites; however, the Kaigas formation was later determined to be non-glacial, and a Sturtian age was assigned to the Numees diamictites.

## Vendian

The Vendian occurred about 740 Ma.
The Vendian concept was formed stratigraphically top-down, and the lower boundary of the Cambrian became the upper boundary of the Vendian.
Paleontological substantiation of this boundary was worked out separately for the siliciclastic basin (base of the Baltic Stage of the Eastern European Platform) and for the carbonate basin (base of the Tommotian Stage of the Siberian Platform).
"During the Vendian–Early Cambrian, a complex system of island arcs (Kuznetsk-Altay island arc system) was situated at the margin of the Siberian continent (Dobretsov et al., 2004, Safonova et al., 2011)."
"The Vendian-Cambrian sequence that crops out along the Katun’ River (northern Gorny Altai, Katun’ zone) is mainly composed of a thick sequence of biogenic carbonate sedimentary rocks that accumulated on shallow marine depositional environments of a basaltic plateau. They belong to two laterally coeval formations, which may reach 1000 m in thickness: the Baratal Formation, made essentially of thick-bedded partly stromatolitc limestones, underlain by black shales, and the Eskongo Formation, made of dark colored dolomites and limestones with some intercalations of chert [...]. These oldest parts of the Katun sedimentary sequence are considered as Vendian to Early Cambrian (Tommotian) in age; the Baratal Formation contains microphytolites of a Vendian age (Buslov et al., 1993, Zybin and Sergeev, 1978). The Eskongo Formation contains microphytolites, calcareous algae and shelly microfauna characteristic of a Vendian-Early Cambrian age (Terleev, 1991). A lot of sponge spicules (Protospongia sp. and Chancelloria sp. and specimens of Monaxonellida, Hexactinellida and Tetraxonida) were also identified in the siliceous levels of this Formation (Zybin et al., 2000)."
The lower boundary of the Vendian was suggested to be defined at the base of the Varanger (Laplandian) tillites.
The Vendian in its type area consists of large subdivisions such as Laplandian, Redkino, Kotlin and Rovno Regional stages with the globally traceable subdivisions and their boundaries, including its lower one.
The Redkino, Kotlin and Rovno regional stages have been substantiated in the type area of the Vendian on the basis of the abundant organic-walled microfossils, megascopic algae, metazoan body fossils and ichnofossils.
The lower boundary of the Vendian could have a biostratigraphic substantiation as well taking into consideration the worldwide occurrence of the Pertatataka assemblage of giant acanthomorph acritarchs.

## Kundelungu

The Kundelungu is dated to 765 ± 5 Ma.

## Sinian

The Sinian spans approximately 800 to 542 Ma.

## Hadrynian

The Hadrynian (North America) spans 850 to 542 Ma.

## Namibian

The Namibian (South Africa) spans 900 to 542 Ma.

## Mesoproterozoic

Def. "a geologic era within the Proterozoic eon; comprises the Calymmian, Ectasian and Stennian periods from about 1600 to 900 million years ago, when the Rodinia supercontinent was formed" is called the Mesoproterozoic.

## Tonian

The Tonian spans 1000 to 850 Ma.
The first putative metazoans (animal) fossils dated to the late Tonian (c. 800 Mya), e.g., the Otavia antiqua, which has been described as a sponge, where dating is consistent with molecular data recovered through genetic studies on modern metazoan species; more recent studies have concluded that the base of the animal phylogenetic tree is in the Tonian.
Def. "a geologic period within the Neoproterozoic era from about 1000 to 850 million years ago" is called the Tonian.

## Karatau

The Karatau spans 1100 to 800 Ma.

## Stenian

The Stenian spans 1200 to 1000 Ma.
Def. the "final geologic period in the Mesoproterozoic Era, from 1200 million to 1000 million years ago" is called the Stenian.
The "Mount John Shale Member of the overlying Wade Creek Sandstone has been dated (Rb/Sr method) at 1.128±0.1 × 10$^9$ yr BP$^1$."

## Adelaidean

The Adelaidean (Australia) spans 1300 to 542 Ma.

## Ectasian

Def. "a geologic period within the Mesoproterozoic era from about 1400 to 1200 million years ago" is called the Ectasian.

## Calymmian

Def. "a geologic period within the Mesoproterozoic era from about 1600 to 1400 million years ago" is called the Calymmian.

## Helikian

The Helikian (North America) spans 1600 to 850 Ma.

## Paleoproterozoic

Def. "a geologic era within the Proterozoic eon; comprises the Siderian, Rhyacian, Orosirian and Statherian periods from about 2500 to 1600 million years ago, when cyanobacteria increased the amount of oxygen in the atmosphere and changed life on Earth for ever" is called the Paleoproterozoic.
Compilation of the reported Palaeoproterozoic diamictite units in the world :
| Continent | Name or strata | Geography or locality | Age (Ma) | References |
| N. America | Gowganda Fm., Cobalt Gp., Huronian SGp. | 45°40′–48°40′ N, 79°–85° W; Ontario, Canada | 2450–2217.5 | Krogh et al. (1984), Andrews et al. (1986) |
| N. America | Bruce Fm., Quirke Lake Gp., Huronian SGp. | 45°40′–48°40′ N, 79°–85° W; Ontario, Canada | 2450–2217.5 | Krogh et al. (1984), Andrews et al. (1986) |
| N. America | Ramsay Lake Fm., Hough Lake Gp., Huronian SGp. | 45°40′–48°40′ N, 79°–85° W; Ontario, Canada | 2450–2217.5 | Krogh et al. (1984); Andrews et al. (1986) |
| N. America | Chibougamau Fm. | 49°40′–50°15′ N, 74°40′–73°50′ W; Quebec, Canada | 2500–1800 | Frakes (1979), Hambrey and Harland (1981) |
| N. America | Padlei Fm., Hurwitz Gp. | 61°–62°30′ N, 95°–99° W; Northwest Territories, Canada | 2300–2100 | Frakes (1979), Hambrey and Harland (1981) |
| N. America | Northern Black Hills | 43°50′–44°07′ N, 103°20′–103°45′ W; South Dakota, USA | 2559–1870 | Dahl et al. (1999) |
| N. America | Bottle Creek, Singer Peak Fm., Snowy Pass Gp. | Snowy Pass Group, Sierra Madre Mountains, Wyoming, USA | <2450 | Frakes (1979), Hambrey and Harland (1981) |
| N. America | Headquarters Fm., Lower Libby Creek Gr., Snowy Pass SGp. | 41°–41°30′ N, 107°15′–106°15′ W, Medicine Bow Mountains, Wyoming, USA | 2451–2000 | Premo and Van Schmus (1989), Cox et al. (2000) |
| N. America | Vagner Fm., Deep Lake Gp., Snowy Pass SGp. | 41°–41°30′ N, 107°15′–106°15′ W, Medicine Bow Mountains, Wyoming, USA | 2451–2000 | Premo and Van Schmus (1989), Cox et al. (2000) |
| N. America | Campbell Lake Fm, Deep Lake Gr., Snowy Pass SGp. | 41°–41°30′ N, 107°15′–106°15′ W, Medicine Bow Mountains, Wyoming, USA | <2451 ± 9 | Premo and Van Schmus (1989) |
| N. America | Fem Creek Fm., Chocolay Gp., Marquette Range SGp. | Menominee and Iron River–Crystal Falls Ranges, Amasa Uplift, WI and MI, USA | 2302–2115 | Bekker et al. (2006), Vallini et al. (2006) |
| N. America | Enchantment Lake Fm., Chocolay Gp., Marquette Range SGp. | 45°49′–46°30′ N, 87°30′–88°05′ W; Marquette Trough, Upper Peninsula Michigan, USA | 2288–2131 | Bekker et al. (2006), Vallini et al. (2006) |
| Africa | Witwatersrand SGp. | South Africa | 2600–2300 | Frakes, 1979; Hambrey and Harland, 1981 |
| Africa | Makganyene Diamictite, Postmasburg Group | 28°47′ S, 23°15′ E; Griqualand West Basin, South Africa | 2415–2222 | Cornell et al. (1996), Gutzmer and Beukes (1998), Bau et al. (1999) |
| Africa | Boshoek Fm, Lower Pretoria Group, Transvaal SGp. | 25°50′ S, 28°25′ E; Transvaal Basin, South Africa | 2316–2249 | Dorland (2004), Hannah et al. (2004) |
| Africa | Duitschland Fm, Lower Pretoria Group, Transvaal SGp. | 25°50′ S, 28°25′ E; Transvaal Basin, South Africa | 2480–2316 | Pickard (2003), Hannah et al. (2004) |
| Australia | Meteorite Bore Mb., Turee Creek Group | 22°55′ S, 117° E; Hamersley basin, Western Australia | 2209–2449 | Barley et al. (1997), Trendall et al. (1998), Pickard (2002) |
| Antarctica | Widdalen Fm. | 71°51′ S, 2°43′ W or 71°05′ S, 2°21′ W | >1700 | Frakes (1979), Hambrey and Harland (1981) |
| Asia | Gangau tillites | 79°07′–79°55′ E, 24°20′–24°40′ N; Central India | 2600–1850 | Frakes (1979), Hambrey and Harland (1981) |
| Asia | Sanverdam tillites | 74°50′–73°10′ E, 15°30′–15°05′ N; South India | 2600–2200 | Frakes (1979), Hambrey and Harland (1981) |
| Europe | Sakukan tillites | Baikal, Russia | 2640–1950 | Melezhik and Fallick (1996), Melezhik et al. (1997b) |
| Europe | Lammos tillites | 68° N, 30° E; Kola Peninsula, Russia | >1900 | Melezhik and Fallick (1996), Melezhik et al. (1997b) |
| Europe | Partanen tillites | Southern Karelia, Russia | 2150–1900 | Melezhik and Fallick (1996), Melezhik et al. (1997b) |
| Europe | Sarioli tillites, Karelian Sgp. | Eastern Baltic Shield, Russia | 2455–2180 | Melezhik and Fallick (1996), Melezhik et al. (1997b) |

## Jinningian

The Jinningian (China) spans 1750 to 800 Ma.

## Carpentarian

The Carpentarian spans 1800 to 1300 Ma.
The "sediments in the McArthur basin (the type locality of the Carpentarian) were deposited between 1800-1400 Ma."
The "microflora of blue-green and green or red algal affinities is from chertified stromatolitic dolomites of the Bungle Bungle Dolomite which outcrops in the Osmond Range of Western Australia."
The Bungle Bungle Dolomite [at least 1.3 km of dolomite, dolomitic shales, shales and sandstones] is considerably [older] and that an age of about 1.5×10$^9$ yr for the microbiota [...] is consistent with the data available."
Radiometric dating has shown that the Kattsund-Koster dyke swarm is about 1421 Ma and the dyke swarm may be related to extensional tectonics.
In the image on the right, the oldest rock is a gray sediment gneiss that was deformed, heated, and partly melted about 1,560 million years ago in the Gothic mountain range (no melted fragments visible in the rock). In its final phase, a gabbromagma penetrated, a mixed rock formed; rounded balls of dark gabbro in light gray gneiss, or bright networks of molten sediment gneiss in the gabbro. A younger gray-white pegmatite passes through the mixed rock at the lighthouses. Walks of the even younger black Koster diabase cut through all rocks.

## Statherian

Def. "a geologic period within the Paleoproterozoic era from about 1800 to 1600 million years ago" is called the Statherian.

## Orosirian

Def. "a geologic period within the Paleoproterozoic era from about 2050 to 1800 million years ago" is called the Orosirian.
On the right is an image of the Vredefort crater, located at 27°0'0"S 27°30'0"E, the largest verified impact crater on Earth, more than 300 kilometres (190 mi) across when it was formed.

## Mokolian

The Mokolian (South Africa) spans 2050 to 900 Ma.
"Currently only two precisely dated Precambrian-age impact structures are known, the 2023 ± 4 Ma, >250 km Vredefort Dome in South Africa$^{8,9}$, and the 1850 ± 1 Ma, >200 km Sudbury structure in Canada$^{10}$."

## Animikean

Animikean General Stage is from 2225 to 1400 Ma.

## Rhyacian

Def. "a geologic period within the Paleoproterozoic era from about 2300 to 2050 million years ago" is called the Rhyacian.
"The ~70 km-diameter Yarrabubba impact structure in Western Australia is regarded as among Earth’s oldest [...] Shock-recrystallised monazite yields a precise impact age of 2229 ± 5 Ma, coeval with shock-reset zircon. This result establishes Yarrabubba as the oldest recognised meteorite impact structure on Earth, extending the terrestrial cratering record back >200 million years."
"The age of Yarrabubba coincides, within uncertainty, with temporal constraint for the youngest Palaeoproterozoic glacial deposits, the Rietfontein diamictite in South Africa."
"Numerical impact simulations indicate that a 70 km-diameter crater into a continental glacier could release between 8.7 × 10$^{13}$ to 5.0 × 10$^{15}$ kg of H$_2$O vapour instantaneously into the atmosphere."
"Yarrabubba is a recognised impact structure located within the Murchison Domain of the Archaean granite—greenstone Yilgarn Craton of Western Australia [...]. No circular crater remains at Yarrabubba; however, the structure has an elliptical aeromagnetic anomaly consisting of an even, low total magnetic intensity domain, measuring approximately 20 km N–S by 11 km E–W [...]. The present day exposure represents a deep erosional level, as neither impact breccias nor topographic expressions of the over-turned rim or central uplift are preserved. Therefore, the ~20 km diameter magnetic anomaly has been interpreted to represent the remnant of the deeply buried central uplift of the structure, which is consistent with an original crater diameter of 70 km$^{18,19}$. Unshocked dolerite dykes formed during either ca. 1200 Ma Muggamurra$^{20}$ or ca. 1075 Ma Warakurna21 regional volcanism cross-cut the elliptical magnetic anomaly and thus post-date the impact event."

## Siderian

Def. "a geologic period within the Paleoproterozoic era from about 2500 to 2300 million years ago" is called the Siderian.

## Aphebian

The Aphebian spans 2500 to 1600 Ma. The Penrhyn Group is a part of the Aphebian.
The Byam Martin Mountains are made up of Archean-Aphebian igneous crystalline rock and Proterozoic metasedimentary and metamorphic rock, such as gneiss with sharp peaks and ridges, divided by deep glacier-filled valleys are typical features in the range.
Nain Archean gneiss is overlain to the north of the community of Nain, Newfoundland and Labrador by the undeformed Ramah Group shale, sandstone and quartzite from the Aphebian.

## Transvaal

The Transvaal is approximately 2700 Ma.

## Neoarchean

Def. "a geologic era within the Archaean eon from about 2800 to 2500 million years ago" or the "era from 2,800 Ma to 2,500 Ma" is called the Neoarchean.

## Randian

The Randian spans about 3000 to 2500 Ma.
"The Witwatersrand Basin formed over a period of 360 Ma between 3074 and 2714 Ma. Pulses of sedimentation within the sequence and its precursors were episodic, occurring between 3086-3074 Ma (Dominion Group), 2970-2914 Ma (West Rand Group) and 2894-2714 Ma (Central Rand Group). Detritus was derived from a mixed granite-greenstone source of two distinct ages; the first comprises Barberton-type greenstone belts and granitoids > 3100 Ma old, and the second consists of the greenstone belt-like Kraaipan Formation and associated granitoids ≤ 3100 Ma old."
"Metamorphism of the Witwatersrand Basin occurred at ca. 2500, 2300 and 2000 Ma. The first two events coincided with the progressive loading of the basin by Ventersdorp and Transvaal cover sequences, whereas the last reflects intrusion of the Bushveld Complex and/or the Vredefort catastrophism."
"The Kaapvaal Craton is the name given to the ancient segment of continental crust which formed in southern Africa between about 3.7 to 2.7 Ga. Much of this continental nucleus actually formed prior to 3.1 Ga by the formation of an extensive granitoid basement and amalgamation with arc-like oceanic terranes represented by mafic/ultramafic volcanics and associated sediments (De Wit et al., 1992)."
"The minimum age of Witwatersrand deposition is provided by the age of the Ventersdorp lavas which immediately overlie the sequence over wide areas. The lower basaltic portion of these lavas has been dated at 2714 ± 8 Ma, whereas higher up in the succession quartz porphyries have been dated at 2709 ± 4 Ma [...]."
"The Crown lava, which occurs just beneath the West Rand- Central Rand Group transition about half way up the sequence, has been provisionally dated at 2914 ± 8 Ma [...], a figure which provides approximate maximum and minimum constraints on Central Rand and West Rand Group deposition, respectively. Detrital zircon grains from the Orange Grove quartzite and the Promise reef, both in the West Rand Group, range in age between 3330 ± 5 and 2970 ± 3 Ma; the latter date represents a maximum constraint on the depositional age of the West Rand Group, with the minimum age determined by the Crown lava at 2914 Ma [...]."
"Detrital zircon grains from conglomerate reefs in the auriferous Central Rand Group become progressively younger upwards in the stratigraphy and the youngest grains occur in the Elsburg and Ventersdorp Contact (VCR) reefs (2894 ± 10 and 2780 ± 5 Ma, respectively). The VCR must, therefore, have been laid down sometime after 2780 Ma ago, but before or at 2714 Ma which is the age of the Ventersdorp lavas (Armstrong et al., 1991)."
"Dominion sedimentation occurred over a relatively brief interval some time after 3086 Ma but before 3074 Ma ago."
"West Rand Group deposition commenced subsequent to 2970 Ma and, consequently, a significant hiatus of some 100 million years appears to exist between the Dominion and West Rand Groups. The latter pulse of sedimentation was largely complete by 2914 Ma, when the Crown lava was extruded, The onset of Central Rand Group deposition commenced after 2914 Ma but may have started as late as 2894 Ma, the age of the youngest detrital zircon grain in the Elsburg reef [...]."
"The Gaborone granite and Kanye rhyolites were emplaced at 2785 ± Ma in an interlude which may have stimulated Turffontein Subgroup deposition (i.e the upper portion of the auriferous Central Rand Group; [...]), although no detrital zircons of this age have yet been found in the latter sequence."

## Mesoarchean

Def. "a geologic era within the Archaean eon from about 3200 to 2800 million years ago; stromatolites have existed from this time" or the "era from 3,200 Ma to 2,800 Ma" is called the Mesoarchean.
"The oldest record of impacts on Earth are Archaean to Palaeoproterozoic ejecta deposits found within the Kaapvaal craton of southern Africa and the Pilbara Craton in Western Australia, spanning ca. 3470 (ref. $^6$) to 2460 Ma$^7$; however, no corresponding impact craters have been identified."

## Paleoarchean

Def. "a geologic era within the Archaean eon from about 3600 to 3200 million years ago; the first aerobic bacteria appeared at this time" or the "era from 3,600 Ma to 3,200 Ma" is called the paleoarchean.

## Archean

Archaean is an alternate spelling of Archean.
Def. "the geologic eon from about 3,800 to 2,500 million years ago; comprises the Eoarchean, Paleoarchean, Mesoarchean and Neoarchean eras; marked by an atmosphere with little oxygen, the formation of the first continents and oceans and the emergence of simple life" or the "eon from 2,500 Ma to 4,000 Ma" is called the Archaean, or Archean.
"About 3 billion years ago, in the Archean eon, the emergence of plate tectonics moved thin and small rafts of rocks at a rate much faster than they do today. Over time, the volcanic islands butted together into large blocks of continental crust, with most of the action (in what is today’s Canada) starting around the Red Lake area, a prolific gold-mining camp in northwest Ontario."
"During the Archean and Proterozoic, the cratons [image on the right] that would soon form the core of Canada were stitched together along vast mountain chains. The rocks, which were made up mostly of greenstone belts and intrusives, were variably metamorphosed and eroded, and the margins of the continents were blanketed with sediments. Orogenic gold mineralization, banded-iron formations, magmatic nickel-copper, volcanogenic massive sulphide and sedex deposits predominated. Uranium mineralization fluxed into major structures during near the end of the Proterozoic."
Def. the "period of time determined to exist prior to 2.5 billion [thousand million] years ago" is called the Archaeozoic or Archeozoic.

## Isuan

The Isuan spans approximately 3800 to 3500 Ma.
Isuan Period – 3810–3490 MYA – named after the Isua Greenstone Belt.
"The TTG basement of the Ancient Gneiss Complex of Swaziland contains minor relics which are as old as 3650-3500 Ma (Compston and Kröner, 1988) and, therefore, predate the Barberton greenstone belt."

## Swazian

The Swazian spans approximately 4000 to 3000 Ma.
The Swazian is a poorly defined geological stage in South Africa extending from about four billion years ago to 3 billion years ago, encompassing some of the Hadean and much of the Archean on the Geologic time scale. Other scales assign the Swazian to parts of the Paleoarchean and Mesoarchean, 3.5 to 2.8 billion years ago.
"The best exposed portion of the Archaean basement on the Kaapvaal Craton is the Barberton region and Swaziland to the east of the Witwatersrand Basin [...], where detailed study over several decades has provided an extensive data base for the study of early crustal evolution. The region is now known to comprise a collage of amalgamated terranes each of which consists of tonalite-trondhjemite-granodiorite (TTG) gneisses and an associated assemblage of metavolcano-sedimentary supracrustal rocks (Lowe, 1994). These terranes were mainly formed at ca. 3480-3440 and 3250-3220 Ma (Armstrong et al., 1990; De Ronde and De Wit, 1994; Kamo and Davis, 1994), although isolated remnants of TTG material and associated metavolcanics in the southwestern portion of the belt are as old as 3550-3530 Ma (Kröner et al., 1991 )."
The "Westerdam granite (3086 ± 3 Ma) was emplaced just prior to Dominion sedimentation, the Coligny granite (3031 ± 11 Ma) was emplaced prior to West Rand Group deposition and the Schweizer-Reneke granite (2880 ± 2 Ma) may have preceded Central Rand Group sedimentation [...]."

## Azoic

Def. destitute "of any vestige of organic life, or at least of animal life; anterior to the existence of animal life; formed when there was no animal life on the globe" is called the azoic.
The Azoic "stands as the first [age] in geologic history, whether science can point out unquestionably the rocks of that age or not." When fossils had been found in strata which had previously been classified as Azoic, the boundary was simply moved lower. "Such changes are part of the progress of the science."

## Hypozoic

Def. "older than the lowest rocks which contain organic remains" is called hypozoic.
"In Scandinavia, as well as along the Alps, and Grampians, hypozoic strata are predominant, while mica schist and gneiss are rare in the Harz, Cornwall, and Wales."

## Eoarchean

Def. "a geologic era within the Archaean eon from about 4600 to 3600 million years ago; the first single-celled life began at this time" or the "era from 4,000 Ma to 3,600 Ma" is called the Eoarchean.
"Greenland greenlandite is part of a 3.8 billion year old, highly metamorphosed succession of rocks. These represent the oldest known supracrustal rocks on Earth (the oldest crustal Earth rocks include 4.03 billion year old Acasta Gneiss, 4.28 b.y. rocks from the eastern Hudson Bay area, and 4.45-4.55 b.y. rocks in the subsurface of Baffin Island, Canada)."
The rock, in the images at left and right, a tonalite gneiss, of the Acasta Gneiss exposed on an island about 300 kilometres north of Yellowknife in the Slave craton in Northwest Territories, Canada, was metamorphosed 3.58 to 4.031 billion years ago and is the oldest known intact crustal fragment on Earth.
The metamorphic rock exposed in the outcrop of the Acasta Gneiss Complex, northwestern Canada, was previously a granitoid that formed 4.2 billion years ago, an age based on radiometric dating of zircon crystals at 4.2 Ga.

## Hadean

Contrast-enhanced true color satellite image (Landsat 5) of the Jack Hills in Western Australia. The rocks of the Jack Hills are very old (~ 3 Billion years) and belong to the so-called Yilgarn Craton. Yet some of these rocks, the Jack Hills conglomerate (actually a metaconglomerate) contain individual grains of zircon which are even older, more than 4 Billion years old. They are used by geoscientists to reconstruct the very early history of the earth.
Def. "the geologic eon from about 4,600 to 3,800 million years ago; marked by the formation of the solar system, a stable Earth-Moon orbit and the first rocks" or the "eon before 4,000 Ma" is called the Hadean.

## Hypotheses

* Each time frame or span of time in geochronology has at least one dating technique.
* Late Proterozoic and Upper Proterozoic are different time frames.
* The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).