An understanding of the origin of clay mineral zones in coastal plain sediments is a prerequisite for the effective stratigraphic application of clay petrology. One common type of clay mineral zone is developed by the occasional migration of a major delta lying near the margin of two clay-petrologic provinces. As an active delta migrates from place to place along a coast, the magnitude of its detrital contribution to any location will vary between a maximum and a minimum. By this process the regressive deltaic phase sediments may alternate with transgressive marine zones in which locally derived or longshore drift material is relatively more prominent.

If the major stream is supplying a clay mineral suite which differs from the longshore current-borne and locally derived clay (i.e., from local weathering, reworking of older beds, small river influx, etc.), a series of correlative regressive-transgressive clay mineral zones will be built up. This process is illustrated by comparing clay mineral variations in cores from the eastern Mississippi Delta area with the known deltaic history of the region.

The abandoned St. Bernard Subdelta, which the Mississippi occupied from 1,700 to 2,800 years ago, has a clay mineral zone sequence similar to that in the sediments of interdeltaic Mississippi Sound which lies 40 miles east. This similarity allows the two sections to be correlated across facies boundaries so that the Recent chronology of the two areas can be related.