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Areal distributions of clay suites of the central Gulf of Mexico area have been studied mainly by X-ray diffraction, using peak-height ratios as an index to the nature of clays. This approach alone yields little information regarding effects of depositional environments. The present study couples geochemical techniques with analysis of other environmental indicators and emphasizes effects of organic and inorganic processes associated with deposition and early diagenesis. Undisturbed samples from both active environments and deep borings were utilized. Samples from freshwater alluvial valley and upper deltaic-plain environments were compared with others from brackish and saline environments of the lower delta. Laboratory techniques included X-ray radiography, X-ray radiography, X-ray d ffraction, X-ray spectrography, atomic absorption spectrophotometry, and microsteam distillation.

Clays deposited in channel and natural levee environments show little modification from the basic transported suite carried by the Mississippi (predominantly montmorillonite with appreciable amounts of illite and kaolinite). Minor differences attributed to seasonal fluctuations in flow regimes of major tributaries are detectable in suspended load and bottom samples. Percent organic C and cation-exchange capacities tend to decrease toward the river mouth.

In the lower delta, properties of delta-front and prodelta clays show distinct gradients, reflecting velocity decrease, water-depth increase, salinity increase, and mixing with shelf clays. Clays become more kaolinitic, less calcium saturated, and more sodium saturated seaward from distributary mouths. In these environments high soluble salt content is associated with a distinct texture and lamination produced by salt flocculation.

Integrated intensity ratio logs of montmorillonite-plus-illite/kaolinite from deep lower-delta borings not only exhibit trends related to delta progradation, but also show clay-mineral zonation resulting from shifting delta lobes. This zonation has been useful particularly in solving stratigraphic problems associated with diapiric mud-lump structures.

Clays are introduced into upper deltaic plain fresh-water basins largely by overbank flow from the main distributaries. Paludal and lacustrine environments are characterized by low rate of detrital introduction, high rates of organic and inorganic chemical deposition, high pH (>8.0), and warm, stagnant water. Despite the fact that secondary minerals (siderite, calcium carbonate, pyrite, vivianite, etc.) in the form of nodules and cementing agents are abundant and form rapidly, the basic nature of the clay remains unaltered even after burial for several thousand years.

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