Three clay mineral zones have been recognized in cores from Recent sediments of the eastern Mississippi Delta area. The clay mineral suite in each of these zones is apparently related to the position of the active Mississippi Delta as follows: (1) during the regressive St. Bernard subdelta phase, when the active delta was near the cored locations, the clay suite deposited there was richly montmorillonitic, reflecting the type of clay being carried by the ancestral Mississippi River; (2) when the site of active deltaic deposition shifted to the west of the cored locations (in this case downcurrent), local transgressive conditions developed in the eastern Delta area, and the clay included in the sediments deposited there was influenced more by local weathering conditions an by longshore drifting of clay from the more kaolinitic eastern Gulf province. These two factors probably combined to form the slightly more kaolinitic clay mineral suite which characterizes the two transgressive phases above and below the sediments of the St. Bernard subdelta.

Comparison of St. Bernard subdelta clays with modern Mississippi River clays indicates that the clay mineral suite carried by the river has become less montmorillonitic within the past few thousand years. The possibility exists, therefore, that the various ancient Mississippi River subdeltas may be characterized by distinctive clay mineral suites.

The highly montmorillonitic clay carried by the St. Bernard phase of the Mississippi River affected, to a limited degree, an area to the east outside of the actual subdelta. This wider distribution, which was probably brought about by oceanic, wind, and tidal currents, extends across some environmental boundaries and has allowed the Recent section of the Mississippi Sound to be subdivided and correlated chronologically with the St. Bernard subdelta section approximately 40 miles away.

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