GEOLOGICAL TIMESCALE

The geological timescale is subject to fine-tuning a) as time-stratigraphic boundaries become formally fixed in sediments/rocks by international agreement, and b) as new data are obtained that bear on the age of specific boundaries. The following compilation of ages of the lower boundaries of the most commonly used intervals is drawn primarily from Gradstein et al. (2004). See http://www.stratigraphy.org/cheu.pdf. The abbreviation Ma refers to ages in millions of years; and ka, to thousands of years. An alternative subdivision of the Cenozoic Era into Paleogene and Neogene periods is now widely used, particularly in marine successions. Paleogene encompasses Paleocene, Eocene and Oligocene epochs; and Neogene, Miocene and younger epochs. The Pennsylvanian and Mississippian are normally combined as the Carboniferous Period outside the United States.  The Ediacaran Period is newly defined and ratified by the international community (Knoll et al., 2004). During the excursion, we are going to see examples of geology from nearly every interval shown from the Mesoproterozoic onwards. So keep this crib handy.

Phanerozoic Eon
Cenozoic Era
      Quaternary Period        
              Holocene Epoch      0.0118 Ma (11.8 ka)    
      Pleistocene   1.806 Ma      
  Tertiary Period        
      Pliocene Epoch   5.332 Ma    
      Miocene   23.03 Ma    
      Oligocene Epoch   33.9 Ma    
      Eocene   55.8 Ma  

 
      Paleocene   65.5 Ma    

Mesozoic Era
  Cretaceous Period   145.5 Ma    
  Jurassic   199.6 Ma    
  Triassic   251.0 Ma    
Paleozoic Era
  Permian Period   299.0 Ma    
  Pennsylvanian   318.1 Ma    
  Mississippian   359.2 Ma    
  Devonian   416.0 Ma    
  Silurian   443.7 Ma    
  Ordovician   488.3 Ma    
  Cambrian   542.0 Ma    
 
Proterozoic Eon

Neoproterozoic Era
 

Ediacaran Period

   635 Ma   Condon et al. (2005), Zhang et al. (2005)
  Cryogenian   850 Ma   Cowie et al. (1989)
  Tonian   1,000 Ma   Cowie et al. (1989)
Mesoproterozoic Era   1,600 Ma   Cowie et al. (1989)
Paleoproterozoic Era   2,500 Ma   Cowie et al. (1989)
         

Archean Eon

   4,558 + 2/-1 Ma   Allègre et al. (1995)
               

Periods of the Paleozoic and Mesozoic eras are further subdivided into epochs, ages and chrons on the basis of included fossils. Subdivision of the Proterozoic and Archean eons is comparatively coarse because rocks of this age are poorly fossiliferous and much less abundant in outcrop.

REFERENCES:

Allègre, C.J., Manhès, G., and Göpel, C., 1995, The age of the Earth: Geochimica et Cosmochimica Acta, v. 59, p. 1445-1456.

Condon, D., Zhu Maoyan, Bowring, S., Wang Wei, Yang Aihua, and Jin Yugan, 2005, U-Pb Ages from the Neoproterozoic Doushantuo Formation, China: Science, v. 308, No. 5718, p. 95-98.

Cowie, J.W., Ziegler, W., and Remane, J., 1989, Stratigraphic Commission accelerates progress, 1984 to 1989: Episodes, v. 12, p. 79-83.

Grastein, F.M., Ogg, J.G., and Smith, A.G., eds., 2004, A Geologic Time Scale 2004: Cambridge, UK, Cambridge University Press, 589 p. (http://www.stratigraphy.org/)

Knoll, A.H., Walter, M.R., Narbonne, G.M., and Christie-Blick, N., 2004, A new period for the geologic time scale: Science, v. 305, p. 621-622.

Zhang Shihong, Jiang Ganqing, Zhang Junming, Song Biao, Kennedy, M.J., and Christie-Blick, N., 2005, U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations: Geology, v. 33, p. 473-476.