The Mississippi Delta is building because the rate at which sediments are supplied is faster than the combined effect of the rate at which they are removed by waves and currents and the rate the delta area is subsiding. River load is discharged into a variety of depositional environments in the Gulf of Mexico. These environments are inter-related, gradational, and persist both laterally and through some depth of water. Sedimentary deposits being formed in these environments have different lithologic and petrographic characteristics but also are related and gradational. Differences between environmental factors and processes produce the lithologic and petrographic differences and also produce correlative differences in depositional rates. Continuing delta growth causes seaward migration of these environments and leads to the formation of deposits, or sediment units, of large areal extent which bear some predictable relationships to each other.

Sedimentary units which are almost entirely the products of river-borne material in order of seaward occurrence are: (1) “marsh deposits,” (2) “delta front silts and sands,” (3) “prodelta silty clays,” and (4) “offshore clays.” These units are deposited approximately concentric to distributary mouths and are analogous to the topset, forset, and bottomset beds of classic delta description. Their characteristics depend to a large extent on the distance from source and depth of water. Another unit, “marginal deposits,” generally occurs between the deltaic clays and outlying sands from marine sources. Marginal deposits have characteristics transitional between the clays and the outlying sands.