# Basin Groups

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**Basin Groups** refers to 9 subdivisions of the lunar [Pre-Nectarian](/source/Pre-Nectarian) [geologic period](/source/Lunar_geologic_timescale).

## Definition

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](/source/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](/source/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](/source/Pre-Nectarian) period.

## References

1. Don Wilhelms (1987). [*Geologic History of the Moon*](http://ser.sese.asu.edu/GHM/). U.S. Geological Survey Professional Paper 1348.

1. 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](https://ui.adsabs.harvard.edu/abs/2001SSRv...96....9S). [doi:10.1023/A:1011937020193](https://doi.org/10.1023/A:1011937020193). [S2CID 117660264](https://api.semanticscholar.org/CorpusID:117660264)

1. R. Korotev; J. Gillis; L. Haskin; & B. Jolliff (2002). "On the age of the Nectaris basin". *Workshop on Moon Beyond*

## External links

- [GeoWhen Database](https://web.archive.org/web/20130106085716/http://www.stratigraphy.org/bak/geowhen/index.html)

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Adapted from the Wikipedia article [Basin Groups](https://en.wikipedia.org/wiki/Basin_Groups) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Basin_Groups?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.