11.1: Transgressions and Regressions
Definitions
Changes in the relative sea-level history of an area are recorded in the geologic record by transgressions and regressions. If we had a time machine and were watching what happened in a region or specific location, we could define these terms based on the direction of shoreline movement (landward or seaward), changes in water depth (getting shallower or deeper). A transgression occurs when the shoreline moves in a landward direction and an overall deepening takes place. A regression occurs when the shoreline moves in a seaward direction and overall shallowing takes place.
Walther’s Law
Walther’s Law states that “facies that occur in conformable vertical successions also occurred in laterally adjacent environments.” In the diagram provided above, we have three laterally adjacent environments: the shoreface, offshore transition, and shelf - our studies of the modern confirm this sequence. If all you had was Core 1, you'd see that these facies are stacked atop one another and that things got progressively deeper through time. Given that, it would be reasonable to interpret this as a confirmable succession that records a transgression. In Core 2, we see exactly the opposite ... the facies succession records progressive shallowing which could be reasonably interpreted as a regression in a conformable succession.
Remember that the presence of unconformities violates Walter’s Law and can place facies atop one another that were nowhere near each other in a lateral sense.