America's Volcanic Past -
Louisiana

The surface of Louisiana is underlain by geologically young sedimentary sequences that were deposited in or adjacent to rivers and deltas in a coastal plain setting. These deposits and those in other states flanking the Mississippi valley indicate that a major river system corresponding to the Mississippi has persisted at least since the Gulf of Mexico began to form by the separation of North America from South America. In general, the entire suite of fluvial, deltaic, and coastal deposits has advanced farther into the Gulf through time, and continues now to fill it with sediment. Evaporation of the shallow sea that was the early Gulf of Mexico produced thick salt deposits, now deeply buried, from which salt domes intrude the overlying strata. Most surface exposures in Louisiana consist of Quaternary sediment.

Louisiana's Oldest Rocks
The oldest exposed rocks in Louisiana are small patches of Late Cretaceous marine rocks that outcrop along the edges of the Prothro and Rayburns salt domes in Bienville Parish. At both salt domes, pits excavated for the extraction of agricultural lime exposed these rocks. Cretaceous fossils have reportedly been collected from the vicinity of the King salt dome in Bienville Parish. However, the outcrops that produced these fossils have been lost. Only the Late Cretaceous strata at the Rayburns salt dome has been studied in detail. Where exposed in a borrow pit, these often highly fossiliferous strata consist of blue-gray marls and chalks, hard white chalk, greensand, gray chalky marl, and olive-gray marl. The lithology of these strata and the fossils found in them indicate that they consist of formations that also outcrop in Arkansas. About 70 to 82 million years ago these strata accumulated at the bottom of a shallow sea that covered Louisiana. The next oldest strata consist of a small outcrop in Caddo Parish, and consist of marine mudstones exposed along the southeast edge of Caddo Lake. They accumulated about 55 to 66 million years ago.


Excerpts from: Louisiana Geological Survey Website, 2001

The Atlantic Plain:³
The Atlantic Plain is the flattest of the provinces. It stretches over 2,200 miles in length from Cape Cod to the Mexican border and southward another 1000 miles to the Yucatan Peninsula. The Atlantic plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf. This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era.