Shallow epicontinental seas transgressed across the Laurentian (North American) craton during the Early Paleozoic as the glaciers melted and sea levels rose. The seas regressed as the glaciers enlarged and sea levels dropped.
The transgression and regression of the seas deposited sequences of sedimentary rocks that reflect the deepening and shallowing of the waters. These are called transgressive-regressive sequences.
Wave-washed sands, muds, and carbonates were deposited in the shallow epicontinental seas. The epicontinental seas were sites of major diversification of marine life.
During regressions, the former seafloor was exposed to erosion, creating extensive unconformities that mark the boundaries between the transgressive-regressive sequences.
The unconformities can be used to correlate particular sequences from one region to another. The unconformity-bounded sequences are sometimes called cratonic sequences.
Two major transgressions occurred during the Early Paleozoic in North America:
See diagram below for full set of cratonic sequences in North America and their timing.
Cratonic sequences of North America. Green areas represent where sedimentary sequences were deposited. They are separated by major unconformities (yellow) where sediment was eroded (or not deposited).
Note that the unconformities cover a greater time range near the center of the craton. Unconformities near the edge of the craton span less time, if they are present at all. This is because the edges of the craton are most likely to remain flooded. The center of the craton is flooded only during times of major sea level high stands or transgressions.
Similar transgressive-regressive sequences are found on other continents, suggesting that worldwide sea level change caused the transgressions and regressions. These worldwide sea level changes were probably related to glaciations and/or seafloor spreading. During times of rapid seafloor spreading, mid-ocean ridge volcanism displaces sea water onto the continents.
Cratonic sequences correspond to Vail curves of global sea level change. Vail curves are derived from seismic stratigraphic profiles, which permit tracing of unconformities across the craton and into thick continental margin sedimentary rocks.
The Vail curve of global sea level changes.
Compare the Vail curves with the cratonic sequences for North America. Note the fairly close correspondence, particularly for the Paleozoic and Cretaceous.
November 11, 2005