A T-R Unit is an allocyclic transgressive-regressive unit. If you are like me, I had no idea what allocyclic meant. Transgressive is advancing, regressive is receding.

Allocyclic is simply an event/cycle that occurs over a depositional system that affects the entire system. Examples can be a sea level change, a climate change, or some tectonic activity. For example, the rise in oxygen, sea levels, and temperatures during the Pennsylvanian could be considered an allocyclic event.

Another variation, called autocyclic, is a localized change.

T-R units use orders of magnitude for measurement. Most of the Pennsylvanian and part of the Permian period is considered a second-order T-R unit.

Orders and Time

The T-R Unit’s magnitude, or order, also helps define the length of geological time. These are:

MagnitudeLength of Time
1st-order225 – 330 million years
2nd-order 65 million years
3rd-order 8 – 10 million years
4th-order 800 – 1500 thousand years
5th-order 400 – 450 thousand years
6th-order100 – 225 thousand years

The Glenshaw Formation comprises 11-12 6th order T-R units, making the age about 5 million years old.


  • 1984 Busch, R.M., Rollins, H.B., Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units