|Divisions of Lunar geologic time :|
|Pre-Nectarian - Nectarian - Early Imbrian - Late Imbrian - Eratosthenian - Copernican|
The Pre-Nectarian Period of the lunar geologic timescale runs from 4533 million years ago (the time of the initial formation of the Moon) to 3920 million years ago, when the Nectaris Basin was formed by a large impact. It is followed by the Nectarian period. Pre-Nectarian rocks are rare in the lunar sample suite; they are mostly composed of lunar highlands material which have been heavily churned, brecciated, and thermally affected by subsequent impacts, particularly during the Late Heavy Bombardment that marks the approximate beginning of the Nectarian Period. The primary Pre-Nectarian lunar highland material is dominated by the rock type anorthosite, which suggests that the early stage of lunar crustal formation occurred via mineral crystallization of a global magma ocean.
This geologic period has been informally subdivided into the Cryptic Era (4.533 - 4.172 Ga ago) and Basin Groups 1-9 (4.172 - 3.92 Ga ago), but these divisions are not used on any geologic maps.
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 to subdivide the unofficial terrestrial Hadean eon. In particular, it is sometimes found that the Hadean eon is subdivided into the Cryptic era, Basin Groups 1-9, Nectarian and Lower Imbrian, though the first two of these lunar divisions are informal and collectively make up the Pre-Nectarian.
- Don Wilhelms (1987). Geologic History of the Moon. U.S. Geological Survey Professional Paper 1348.
- W. Harland, R. Armstrong, A. Cox, L. Craig, A. Smith, D. Smith (1990). A Geologic time scale 1989. Cambridge University Press.