The division of these sequences were originally constructed in the
Appalachian Basin and
Cordilleran Basin of North America. Eventually these sequences were correlated in Russia and South America. The transgressive-regressive units show gaps in the rock records which indicate times of continual
erosion and very little deposition. Local tectonics did not play a role in these global events, rather, a worldwide rise and fall of sea level (which is not to say that local tectonics have no influence on how the global events are expressed locally). Smaller orders in stratigraphic cycles have also been proposed. Fifth-order cycles and sixth order cycles have also been described in much of the
Absaroka sequence. The time scale is much smaller and instead of
Wilson cycle controlled sea-level change, these shorter cycles were controlled by
glaciers (also called glacio-eustasy).
First-order cycles This cycle is most likely caused by the break-up and formation of super-continents. The earth went through major climatic swings over the course of 200 to 400 million years. From the late
Pre-Cambrian to the late
Cambrian, late
Devonian to the
Triassic-
Jurassic border, and since the
Miocene until the present time, the earth was an "icehouse", with ice sheets covering the poles. In the intervening years, the earth was a "greenhouse", with high global temperatures and elevated atmospheric CO2. Volcanic activity was also high in the greenhouse years. During times of increased sea floor spreading, more
magma is being produced and the volume of the ocean basins are displaced by this. This would result in a higher sea level. This increase in magmatic activity corresponds to increased mantle activity and the Earth's
magnetic field. Another theory is that earth's true
polar wander occurs over a long period of time. The
tectonic plates of the earth would move relatively faster due to imbalance of continents near the poles. This was true during the Cambrian Period, but the same event also happened approximately 66 million years ago but not as severely.
Third-order cycles This order of sea-level change has yet to be fully explained. It was originally thought that glaciers controlled these sea-level changes. But
glaciers form and retreat far too rapidly, only tens of thousands of years instead of over a million years. Instead, short-term changes in earth's surface due to volcanics and tectonic events could change global sea levels over a million years. This change to earth's shape could produce "bulges" or "sags" that contribute to ocean level fluctuations.
Fourth order cycles Again, there are two competing theories for what controls fourth order cycles. Often called
cyclothems, the relative short time period in which individual layers of rock are never more than 1 million years. Glaciers are capable of causing quick changes in sea level that can show up in the rock record. This mechanism has been proposed for many of the
Carboniferous-aged coal deposits producing in some regions, such as North America, cycles of seashore advance and retreat of approximately 600 miles along the shallow slopes of the continental margins.
Delta switching has been proposed as an additional mechanism to produce cyclothems. A modern analogue to describe delta switching would be the
Mississippi embayment. As the
Mississippi River carries its sediment load into the
Gulf of Mexico, the delta lobe can become sediment-choked and the river will look for a new channel to follow the
path of least resistance. Once the influx of terrestrial sand and silt stops, the area might subside and marine sediments may dominate. Also, whatever terrestrial plants are there can be buried and could eventually become
coal. The new river channel will now carry the terrestrial sediments to a new delta, once again starting a new cycle of delta-switching. ==Event stratigraphy==