Cored borings of Recent swamp deposits in the Atchafalaya basin presented a difficult problem in deciphering ecological changes. The fauna was either absent or badly leached; grade-size parameters remained nearly constant from top to bottom; and the 100 ft.-cored section consisted of "massive-appearing" clays. X-ray radiographs of core slabs, however, revealed a varied assemblage of primary, secondary, and post-depositional sedimentary structures and epigenetic and syngenetic inclusions.

Five environmentally controlled facies were recognized in the borings; poorly drained (stagnant) swamp; well-drained swamp; fresh-water-lacustrine; lacustrine delta fill; and channel fill. Overlying a cross-bedded sand and gravel substratum (braided channel-fill deposits) is a 10-ft. layer of swamp clay. Large carbonate and iron concretions, finely disseminated iron cement, and replacement of organic fragments by iron oxides attest to high-oxidizing conditions. Near the top of the layer, there is a gradual change to more reducing conditions and stable water levels. As the rate of sedimentation decreased, 22 ft. of highly organic clays was deposited in a poorly drained swamp environment. High pyrite and vivianite content, siderite concretions, high organic layers, and intensive root bu rowing characterize this unit. As sedimentation rates increased, conditions gradually changed to allow better drainage; the accumulated clays are similar to the lowermost clay unit. During accumulation of this 12-ft. sequence, water levels apparently fluctuated considerably, because intensive leaching is apparent. A sharp reduction in sediment supply and compaction of previously deposited clays resulted in the formation of a large lake. Thirteen ft. of "massive" lucustrine clays was then deposited. Radiographs revealed that the lower part is highly burrowed, and few remnants of primary stratification remain. The upper section, however, consists of alternating laminations (0.1-2 mm. thick) of differing X-ray absorption caused from alternating layers of well-oriented clay particles (floccu ated settling), as well as local concentrations of colloidal organics and carbonates. Sedimentation rates gradually increased, and normal lacustrine sedimentation was interrupted by an advancing lacustrine delta. Twelve ft. of alternating silt, silty clay, and clay forms this facies. A fresh-water shell layer marks the final fill of the lake. Poorly drained swamp conditions were established again and apparently continued uninterruptedly, until the present. Subsequent diagenic changes include growth of concretions, gradual change in concretion composition with depth, and enrichment of certain layers in carbonate.

End_of_Article - Last_Page 2321------------