1. Transgressions and regressions: the key to Earth history
Geology 103

2. The oceans were deeper
Rock (principally marine limestones and shales) and fossil evidence indicates that the ocean occupied the interior of North America (as well as other continents)
Conclusion: sea level was higher (change in the elevation of the continent also played a role)
This is a sea level transgression

3. The oceans were shallower

4. Walther’s Law of Facies
Facies = a rock or rocks deposited in a particular environment
Johannes Walther (1894) stated “A vertical change in facies in a sequence is reflected in the horizontal change in facies along a surface.”

5. Facies change during a transgression
As sea level increases in an area, the water energy decreases, allowing the deposition of finer-grained sediments in the same area
This is an example of Walther’s Law

6. Facies change during regressions, too

7. So, at an outcrop, transgressive sequences are distinguishable from regressive sequences

8. Worldwide transgressive sequences are found in the rock record
Indicates that sea level changes occur over the millions of years timescale, so correlated to plate tectonic changes and continent orientations (especially over polar regions)
Why no regressive sequences recorded?

9. Cratonic transgressive sequences are related to orogenic events
Transgressive sequences seem to coincide with periods of significant mountain building
The mountains provide the source of sediment for the sequences