July 15, 2024

Geologic Time

Geologists have a bunch of words for geologic time periods. It is worth remembering that these terms were developed before age dating using radioisotopes put numbers on these time periods.

The following diagram is from a National Park Service website:

Here is my quick guide, including some notes about time in years.

Some big divisions

I tend to think in two big divisions -- Precambrian and everything else. The fancy word for "everything else" is Phanerozoic -- these are rocks with fossils. 
Phanerozoic -- 543 my to present
Precambrian -- 3800 my to 543 my
Hadean -- pre 3800 my
I never hear "Hadean" used. You have to visit the Canadian shield or Australia to see these old rocks. The current estimate for the age of the earth is 4600 my.

Divisions of the Precambrian

In Arizona, a two fold division of the Precambrian is useful. We have younger precambrian (Apache Group in southern Arizona, and the Grand Canyon group up north). We also have older precambrian (Pinal schist and Oracle granite down south, and Vishnu schist in the Grand Canyon.

More formally, a three fold division is used: 

Proterozoic  2500 to 543 my
Archaen  -- 3800 to 2500 my
Hadean -- pre 3800 my
These can be broken up as follows, but rarely are: 
Late Proterozoic -- 900 to 543 my
Middle Proterozoic -- 1600 to 900 my
Early Proterozoic -- 2500 to 1600 my
Late Archean -- 3000 to 2500 my
Middle Archean -- 3400 to 3000 my
Early Archean -- 4600 to 3400 my
You will sometimes hear of "Precambrian X, Y, and Z". This may be a 3-fold division of the Proterozoic, with "Z" being the most recent (sometimes called "Neoproterozoic"). This is a murky nomenclature (and there is also "W") and is not widely used. It enjoyed some popularity with certain workers from the USGS.

Divisions of the Phanerozoic

Few geologists try to sort out events in the Precambrian. Too much has gone on too long ago and has been overprinted by more recent events. It is fairly rare to hear the word "Phanerozoic" used except in the context of a twofold Phanerozoic/Precambrian division.

There are 3 big divisions of the Phanerozoic: 

Cenozoic -- 65 my to present
Mesozoic -- 248 to 65 my
Paleozoic -- 543 my to 248 my
The Paleozoic gets broken up as follows: 
Permian -- 290 to 248 my
Pennsylvanian -- 323 to 290 my
Mississippian -- 354 to 323 my
Devonian -- 417 to 354 my
Silurian -- 443 to 417 my
Ordovician -- 490 to 443 my
Cambrian -- 543 to 490 my
The Mesozoic gets broken up as follows: 
Cretaceous -- 144 to 65 my
Jurassic -- 206 to 144 my
Triassic -- 248 to 206 my
The Cenozoic gets broken up as follows: 
Holocene -- 0.01 my to present
Pleistocene -- 1.8 to 0.01 my
Pliocene -- 5.3 to 1.8 my
Miocene -- 23.7 to 5.3 my
Oligocene -- 33.7 to 23.7 my
Eocene -- 54.8 to 33.7 my
Paleocene -- 65 to 54.8 my
The Holocene and Pleistocene are referred to together as Quarternary.
The rest of the Cenozoic gets referred to as Tertiary.

Sometimes Miocene and Pliocene get lumped together as "Neogene", but this is rare.
Similarly, Paleocene, Eocene, and Oligocene rarely get lumped as "Paleogene".

There is always some individual eager to use some seldom used term in an effort to try to impress you.

Taken from page 14 of "Aerial Geology" by Mary Caperton Morton.

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Tom's Geology Info / [email protected]