The Mississippi Embayment Synclinorium
In Late Cretaceous time, a shallow seaway once crossed North America from the Gulf of Mexico to the Arctic Ocean. Eastern Arkansas consisted of a shallow ocean. This seaway was populated with now extinct clams, snails, sharks, and marine reptiles such as mosasaurs and plesiosaurs.
Sediment from this seaway slowly accumulated in a partial rift of the crust below called the Reelfoot Rift. These sediments consisted of sands, limey muds, and clays. These sediments eventually lithified (solidified) sandstone, limestone and shale.
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