ABSTRACT

Cored borings of Recent swamp deposits that accumulated in the Atchafalaya basin presented a difficult problem in deciphering ecological changes. Faunal content was either absent or badly leached; grade size parameters remained nearly constant from top to bottom; and the 100 foot 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, as well as 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 ten foot layer of swamp clays. Large carbonate and iron concretions, finely disseminated iron cement, and replacement of organic fragments by iron oxides all attest to high oxidizing conditions. Near the top of the layer, there is a gradual change to more reducing conditions and stable water levels. Rate of sedimentation reduced and 22 feet of highly organic clays were deposited in a poorly drained swamp environment. High pyrite and vivianite content, siderite concretions, high organic layers, and intensive root burrowing 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 foot sequence, water levels apparently fluctuated considerably as 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 feet of "massive" lacustrine clays were then laid down. Radiographs revealed that the lower portion is highly burrowed, and few remnants of primary stratification remain. The upper section, however, consists of alternating lamina tions (0.1 to 2 mm in thickness) of differing X-ray absorption caused from alternating layers of well-oriented clay particles (flocculated 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 feet of alternating silts, silty clays, and clays form this facies. A fresh-water shell layer marks the final fill of the lake. Poorly drained swamp conditions were once again established and continued apparently uninterrupted until the present, Subsequent diagenic changes include growth of concretions, gradual change in concretion composition with depth, and enrichment of certain layers in carbonate.