megasequence

A megasequence is a very large-scale cycle of deposition and erosion that records a significant change in relative sea level over millions of years. These cycles are recognized in sedimentary rocks and represent fundamental units in chronostratigraphy, the study of rock strata in relation to time. The concept of megasequences was developed by researchers studying the sedimentary record of the North American craton. These large depositional packages are bounded by unconformities, which are surfaces representing periods of erosion or non-deposition, indicating significant shifts in sea level. Each megasequence typically comprises a transgressive phase, where sea level rises and sediment deposition occurs on a widening shelf, followed by a regressive phase, where sea level falls and deposition shifts to narrower areas or is replaced by erosion. The identification and mapping of megasequences help geologists understand regional and global sea-level history and correlate rock units across vast distances. They are a key tool in deciphering the Earth's geological past and reconstructing ancient environments. The duration of a megasequence can span tens of millions of years, reflecting major tectonic or climatic events that influence long-term sea-level fluctuations.