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:
| Magnitude | Length of Time |
| 1st-order | 225 – 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-order | 100 – 225 thousand years |
The Glenshaw Formation comprises 11-12 6th order T-R units, making the age about 5 million years old.
References
- 1984 Busch, R.M., Rollins, H.B., Correlation of Carboniferous strata using a hierarchy of transgressive-regressive units