Precambrian - Subdivisions

Subdivisions

An established terminology has evolved covering the early years of the Earth's existence, as radiometric dating allows plausible real dates to be assigned to specific formations and features. The Precambrian Supereon is divided into three Precambrian eons: the Hadean (4500-3950 Ma), Archean (4000-2500 Ma) and Proterozoic (2500-541.0 ± 1.0 Ma). See Timetable of the Precambrian.

• Proterozoic: this eon refers to the time from the lower Cambrian boundary, 541.0 ± 1.0 Ma, back through 2500 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary. The Proterozoic eon is divided into three eras: the Neoproterozoic, Mesoproterozoic and Paleoproterozoic.
  • Neoproterozoic: The youngest geologic era of the Proterozoic Eon, from the Cambrian Period lower boundary (541.0 ± 1.0 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American geology.
    • Ediacaran: The youngest geologic period within the Neoproterozoic Era. The "2012 Geologic Time Scale" dates it from 541.0 ± 1.0 to ~635 Ma. In this period the Ediacaran fauna appeared.
    • Cryogenian: The middle period in the Neoproterozoic Era: ~635-850 Ma.
    • Tonian: the earliest period of the Neoproterozoic Era: 850-1000 Ma.
  • Mesoproterozoic: the middle era of the Proterozoic Eon, 1000-1600 Ma. Corresponds to "Precambrian Y" rocks of older North American geology.
  • Paleoproterozoic: oldest era of the Proterozoic Eon, 1600-2500 Ma. Corresponds to "Precambrian X" rocks of older North American geology.
• Archean Eon: 2500-4000 Ma.
• Hadean Eon: 3950-4500 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the moon and meteorites.

It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows.

1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
2. Hadean: dominated by heavy bombardment from about 4.51, (possibly including a Cool Early Earth period) to the end of the Late Heavy Bombardment period.
3. Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
4. Transition: a period of continued iron banded formation until the first continental red beds.
5. Proterozoic: a period of modern plate tectonics until the first animals.