Transgressions and regressions: the key to Earth history Geology 103

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

The oceans were shallower

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.”

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

Facies change during regressions, too

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

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?

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