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The clay mineral assemblage in an area of active deposition is dependent largely on the character of the source area. This conclusion is based on a study of the clay mineralogy of sediments of the lower Mississippi River, the active delta region, the St. Bernard Sub-delta, and the Mississippi Sound-Mobile Bay area. Montmorillonite, the predominant clay mineral in the Mississippi River and Delta sediments, is apparently the stable product of soil development and rock weathering in the drainage basin of the Mississippi River. The sediments of the Mississippi Sound-Mobile Bay area, east of the Mississippi Delta and derived ultimately from the Appalachian Province, contain considerably more kaolinite. Transition zones between the two sediment types can be differentiated.
Alteration of clay minerals in a depositional area can be expected where the sedimentation rate is low and sufficient time is available for chemical equilibria between the sea water and the clay minerals. In areas of rapid mud deposition, the blanketing effect of overlying clay material probably reduces the chemical interaction to that which is possible between the clay and the entrapped water.
The Mississippi River sediment carried into the saline environment, does not show significant changes in clay mineralogy except along the shelf edge. In this area of slow sedimentation, illite (clay mica) is more prominent. In the St. Bernard Sub-delta, a non-active depositional area for the last 400 years, alteration or diagenesis of the contained clay minerals appears to be associated with evidence of exposure of the sediment to surface oxidizing conditions. In the Mississippi Sound-Mobile Bay area the clays alter rapidly upon entering the saline environment. A part of the clay mineral assemblage has been attributed to a montmorillonite-organic complex, which appears to be modified or destroyed in the alkaline sea-water environment.
An examination of buried muds of Tertiary age has indicated that little or no alteration of clay minerals takes place in shales to a depth of several thousand feet. However, clay minerals contained in sands may be greatly altered during burial.
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