Basin Groups

9 subdivisions of the lunar Pre-Nectarian geologic period

Basin Groups refers to 9 subdivisions of the lunar Pre-Nectarian geologic period. It is the second era of the Hadean.

## Definition

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The motivation for creating the Basin Groups subdivisions was to place 30 pre-Nectarian impact basins into 9 relative age groups. The relative age of the first basin in each group is based on crater densities and superposition relationships, whereas the other basins are included based on weaker grounds.[1] Basin Group 1 has no official age for its base, and the boundary between Basin Group 9 and the Nectarian period is defined by the formation of the Nectaris impact basin.

The age of the Nectaris basin is somewhat contentious, with the most frequently cited numbers placing it at 3.92 Ga, or more infrequently at 3.85 Ga.[2] Recently, however, it has been suggested that the Nectaris basin could be, in fact, much older and might have formed at ~4.1 Ga.[3] Basin Groups are not used as a geologic period on any of the United States Geological Survey lunar geologic maps. Basin Groups 1-9 and the earlier (informal) Cryptic era together make up the totality of the Pre-Nectarian period.

Millions of years before present

## Relationship to Earth's geologic time scale

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Since little or no geological evidence on Earth exists from the time spanned by the Pre-Nectarian period of the Moon, the Pre-Nectarian has been used as a guide by at least one notable scientific work[4] to subdivide the unofficial terrestrial Hadean eon. In particular, it is sometimes found that the Hadean eon is subdivided into the Cryptic era and Basin Groups 1-9 (which collectively make up the Pre-Nectarian), and the Nectarian and Lower Imbrian. The first lifeforms (self replicating RNA molecules, see RNA world hypothesis) may have evolved on earth around 4 bya during this era.

## See also

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* Hadean eon-related topics
* Cryptic era
* Geological time scale (Earth)

## References

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1. ^ Don Wilhelms (1987). Geologic History of the Moon. U.S. Geological Survey Professional Paper 1348.
2. ^ D. Stöffler and G. Ryder, D.; Ryder, G. (2001). "Stratigraphy and isotope ages of lunar geologic units: chronological standards for the inner solar system". Space Sci. Rev. 96: 9–54. Bibcode:2001SSRv...96....9S. doi:10.1023/A:1011937020193. S2CID 117660264.
3. ^ R. Korotev; J. Gillis; L. Haskin & B. Jolliff (2002). "On the age of the Nectaris basin". Workshop on Moon Beyond: abstract 3029.
4. ^ W. Harland; R. Armstrong; A. Cox; L. Craig; A. Smith; D. Smith (1990). A Geologic time scale 1989. Cambridge University Press.

## External links

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* GeoWhen Database

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Hadean Eon

* Cryptic
* Basin Groups
* Nectarian
* Lower Imbrian

The Hadean does not have any subdivisions recognized by the International Commission on Stratigraphy. These subdivisions represent one proposal that is loosely based on the lunar geologic timescale.

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Geological history of Earth

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(present–66.0 Ma)|

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Mesozoic Era
(66.0–252 Ma)|

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Proterozoic Eon
(539 Ma–2.5 Ga)|

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Mesoproterozoic (1–1.6 Ga)|

* Stenian (1–1.2 Ga)
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Paleoproterozoic (1.6–2.5 Ga)|

* Statherian (1.6–1.8 Ga)
* Orosirian (1.8–2.05 Ga)
* Rhyacian (2.05–2.3 Ga)
* Siderian (2.3–2.5 Ga)

Archean Eon (2.5–4 Ga)|

* Neoarchean (2.5–2.8 Ga)
* Mesoarchean (2.8–3.2 Ga)
* Paleoarchean (3.2–3.6 Ga)
* Eoarchean (3.6–4 Ga)

Hadean Eon (4–4.6 Ga)|

ka = kiloannum (thousand years ago); Ma = megaannum (million years ago); Ga = gigaannum (billion years ago).
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