In sequence stratigraphy, the fundamental events that determine the locations of environments and unconformities are transgressions and regressions.
*A transgression is a landward shift in the coastline, and hence a landward shift in all marginal marine environments. A regression is a seaward shift in the coastline.
*A drop in relative sea level always causes a regression. A transgression hence requires rising relative sea level. However, rising sea-level can result in transgression, stationary shorelines, or regression depending on sediment supply.
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