Our last post showed the Persian Gulf, (http://tinyurl.com/pp6l5fp) a flooded foreland basin sitting in space formed when the Earth’s crust is dragged down by the weight of the Zagros Mountains.
These rocks come from the other side of the world and are hundreds of millions of years older, but they tell the exact same story. These strata outcrop in Kentucky and date back to the time when the Appalachian Mountains were growing. They’ve been tilted and are no longer flat lying, but they were deposited as the Appalachians grew.
The mass of the Appalachian Mountains, created when a series of island arcs and then eventually Africa and Europe ran into North America, were a huge weight on the Earth’s surface. That weight dragged down the Earth’s crust and created a basin running up and down the North American continent.
During much of the time when the Appalachians were growing, sea level globally was higher than today; a consequence of the presence of huge ice sheets in Antarctica and Greenland today. Higher sea levels allowed the oceans to flood onto the North American continent, creating a series of inland seaways behind the mountains, just as the Persian Gulf sits next to the Zagros today.
Cyclothems are packages of sediment created during these processes. When sea level would rise, the sediments deposited in the basin would change. They would start off as on-shore, river sediments. Beach sands and coal layers formed from near-ocean swamps would follow those. Finally, deep-water shales and limestones form as the waters inundated the land.
To create the cycle, the waters would then retreat, leading to erosion and the formation of unconformities. Finally, the waters come back in, repeating the same basic package of sediment over and over.
Sediments just like this cover much of the Eastern United States, as well as areas in Europe.
Image credit: http://www.flickr.com/photos/universalpops/6194337959/i