Cored borings of Recent fresh-water swamp deposits that accumulated in the Atchafalaya River basin, Louisiana, revealed the existence of numerous early diagenetic changes in clay deposits and the formation of various types of syngenetic and epigenetic inclusions. The deposits, approximately 100 feet thick, range in age from contemporary to slightly greater than 10,000 years and were deposited in four major environments of deposition: poorly drained (stagnant) swamp, well-drained swamp, lacustrine, and lacustrine delta fill. Many of these environmentally controlled facies are repeated several times in a single vertical sequence and offer the unusual possibility of studying diagenetic changes at different stages of development within a particular environment. The most commo diagenetic change is the replacement of plant rootlets and other organic fragments by pyrite and calcium carbonate. Pyrite replacement is most common in the poorly drained swamp sediments, whereas carbonate replacement occurs most commonly in the well-drained swamp deposits. These changes took place rapidly, probably within a few years after deposition. Both pyrite and calcium carbonate tend first to be formed in the open spaces within the organic fragments, second to invade the pore spaces, and last to replace most of the original organic material. Vivianite (Fe₃P₂O₈ · 8H₂O) forms fairly rapidly and also replaces organic material.

Nodules are abundant throughout the section, but are more common in lacustrine sediments. The size of the nodules differs, generally being smaller in the younger units, and increases in size in the older units. The shape changes, ranging from round, flattened, lenticular masses (lacustrine) to round and irregular-shaped

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masses (swamp). Although hardness tends to increase with increasing burial depth and age, some nodules in relatively young deposits are very hard. Preliminary chemical analyses and X-ray diffraction studies indicate that they are composed of various carbonates, mainly calcium, manganese, magnesium, calcium-magnesium, and iron. Iron carbonates are abundant and appear to be more common in the older deposits. The change in proportion of the various carbonates presently is being investigated but this change must be abrupt. Other inclusions, such as iron oxides, iron hydroxides, etc., also are abundant.

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