Cratonic sequence

The Western Interior Seaway, illustrated at 95 million years ago, was a result of transgression onto the North American continent during the mid-Cretaceous period. Sediment deposited by this seaway is represented in the rock record by the Zuñi sequence.

A cratonic sequence (also known as megasequence, Sloss sequence or supersequence) in geology is a very large-scale lithostratigraphic sequence in the rock record that represents a complete cycle of marine transgression and regression on a craton (block of continental crust) over geologic time. They are geologic evidence of relative sea level rising and then falling (transgressing and regressing), thereby depositing varying layers of sediment onto the craton, now expressed as sedimentary rock. Places such as the Grand Canyon are a good visual example of this process, demonstrating the changes between layers deposited over time as the ancient environment changed.

Cratonic sequences were first proposed by Laurence L. Sloss in 1963.[1] Each one represents a time when inland seas deposited sediments across the craton. The top and bottom edges of a sequence are each bounded by craton-wide unconformities (time gaps in the rock record). The unconformities indicate when the seas receded and sediment was eroded rather than deposited.

  1. ^ "Lawrence Sloss, "Tectonic Cycles of the North American Craton". (Accessed 6/18/06)". Archived from the original on 2007-03-10.