The study of more than a thousand samples from the Mississippi Delta margins and from shallow borings made into the delta has provided some criteria for recognizing ancient delta facies. Almost all Mississippi Delta sediments are high in wood fibers, mica, and ferruginous aggregates.
It is suggested that this rapidly advancing delta includes the traditional top-set, fore-set, and bottom-set beds, although the depositional slope of the fore-set beds is almost everywhere less than 1°. The top-set beds are divided into subaerial and shallow marine. The subaerial include channel deposits, a mixture of sandy and clayey sediments with rare Foraminifera introduced by the salt wedge which penetrates the river mouth at low water; levee deposits consisting of laminated sands and silty clays; and marsh deposits which include peat beds and an abundance of rootlets along with layers of silty sand.
The shallow marine includes delta-front platform deposits which are commonly laminated and show rapid lateral and vertical changes from silty sand to silty clay, and interdistributary bay deposits which contain almost as much sand as the delta-front platform deposits and differ only in having somewhat thicker layers of clayey sediments and less lamination. The fore-sets beds show a marked change from the platform deposits in having very little lamination and consisting almost entirely of poorly stratified silty clays. The fore-set beds resemble the top-set in having a scarcity of organic remains. The bottom-set beds are also silty clays, but they have a considerably greater percentage of Foraminifera, echinoids, and shell fragments than found in the other environments, evidently owing to slower deposition and higher productivity at a moderate distance beyond the river mouth. Mottling patterns due to reworking by organisms appear in the bottom-set beds and are found to an even greater degree in the more sandy deposits of Breton Sound.
Reworked delta deposits from which the fines have been removed are found in Breton Island and on the sea floor to the east, whereas a sandy old shelf deposit occurs farther east and to the south. These old shelf deposits are being overlapped by the bottom-set beds of the modern delta.