Basin Groups

Basin Groups refers to 9 informal subdivisions of the lunar Pre-Nectarian geologic period.

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.cite book | title = [http://ser.sese.asu.edu/GHM/ Geologic History of the Moon] | last = Don Wilhelms | publisher = U.S. Geological Survey Professional Paper 1348 | year = 1987] 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. [cite journal | last = D. Stöffler and G. Ryder | title = Stratigraphy and isotope ages of lunar geologic units: chronological standards for the inner solar system | year = 2001 | journal = Space Sci. Rev. | pages = 9–54 | volume = 96 | doi = 10.1023/A:1011937020193] Recently, however, it has been suggested that the Nectaris basin could be, in fact, much older and might have formed at ~4.1 Ga. [cite journal | last = R. Korotev, J. Gillis, L. Haskin, and B. Jolliff | title = On the age of the Nectaris basin | journal=Workshop on Moon Beyond | year = 2002 | pages = abstract 3029] 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.

Relationship to Earth's geologic time scale

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 [cite book | title = A Geologic time scale 1989 | publisher = Cambridge University Press | year = 1990 | last = W. Harland, R. Armstrong, A. Cox, L. Craig, A. Smith, D. Smith] 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.

ee also

* Cryptic era
* Geological time scale (Earth)

References

External links

* [http://www.stratigraphy.org/geowhen/index.html GeoWhen Database]